COAL  WASHING 


Sftfe  Qrm*3/illBoak  (a  Jne 

PUBLISHERS     OF     BOOKS      F  O  R^_, 

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COAL  WASHING 


BY 

ERNST  PROCHASKA.  M.E. 

CONSULTING  ENGINEER,   MEMBER  AMERICAN 
INSTITUTE  MINING  ENGINEERS 


FIRST  EDITION 


McGRAW-HILL  BOOK  COMPANY,  INC, 

NEW   YORK:    370   SEVENTH   AVENUE 

LONDON:    6  &  8  BOUVERIE  ST.,  E.  C.  4 

1921 


T1 


Copyright,  1921,  by  the 
McGRAW-HILL  BOOK  COMPANY,  INC. 

r 


THIS  BOOK  IS  DEDICATED 

TO  THE  MEMORY  OF 

THE  LATE 

ELL  WOOD  A.  STEWART 

IN  RECOGNITION  OF  HIS  COURAGEOUS 

PERSISTENCE  AND  ENGINEERING 

SKILL  IN  THE  DEVELOPMENT 

OF  THE  ART  OF  COAL 

WASHING 


435339 


PEEFACE 

This  book  has  been  compiled  for  the  purpose  of  giving  a  sys- 
tematic description  of  modern  practice  in  the  art  of  coal  wash- 
ing. This  may  appear  at  first  glance  to  be  much  easier  than  it 
really  was.  The  question  arose :  Should  preference  be  given  to 
thp  methods  at  present  in  actual  use  or  to  those  which  in  the 
light  of  our  present  experience  are  most  judicious? 

There  will  be  always  a  difference  between  ideal  performance 
and  what  is  actually  accomplished.  A  coal  washery  is  an  ex- 
pensive installation,  built  for  long  service.  During  its  useful 
life  the  art  of  coal  washing  may  advance  uninterruptedly.  The 
necessary  and  justifiable  desire  of  the  operator  to  try  out  all 
useful  inventions,  tends  in  some  degree  to  equalize  the  above 
difference.  New  apparatus  can  be  added  to  an  old  washery,  but 
the  total  reconstruction  of  an  existing  plant,  so  as  to  bring  it 
up  to  modern  practice,  is  hardly  justified,  since  on  account  of 
the  constant  changes  in  our  ideas  and  the  development  of  new 
machinery,  the  remodeling  of  a  plant  would  never  be  finished. 

To  neglect  in  this  description  the  older  types  of  washeries, 
which  are  still  in  operation  and  doing  good  work,  would  put 
the  present  and  future  into  too  prominent  a  place. 

This  book  is  intended  to  furnish  the  coal  operator  with  the 
necessary  knowledge  whereby  he  may  distinguish  in  washeries 
between  the  modern  and  earlier  apparatus  and  methods  which 
may  be  working  side  by  side. 

Everything  is  omitted  concerning  apparatus  which  because  of 
a  lack  of  progressive  spirit  is  still  in  use,  even  though  it  be 
archaic.  Only  a  study  of  the  chronological  development  of  the 
art  of  coal  washing  will  enable  us  to  make  a  judicious  selection 
of  equipment. 

An  effort  is  here  made  to  set  up  a  proper  or  standard  rule  for 
guidance  in  the  present  as  well  as  the  future,  from  a  survey  of 
the  experience  of  the  past,  with  its  mistakes  and  blind  alleys, 
its  roundabout  ways  and  unwarranted  short-cuts.  Only  such 
methods  will  be  omitted  as  have  been  proven  impracticable  and 
useless. 

vii 


yjji  PREFACE 

It  must  be  remembered,  however,  in  connection  with  the  above 
statement,  that  a  good  many  failures  during  the  earlier  stages 
of  the  development  of  coal  washing  were  caused  by  the  lack  of 
proper  technical  training  and  education  on  the  part  of  the  ex- 
perimenters. No  failure  of  a  method  during  the  earlier  periods 
of  coal  washing  can  be  considered  as  being  complete  either  at 
present  or  in  the  future. 

The  present  state  of  coal  washing  is  the  result  of  a  steady 
evolution.  A  comprehensive  description  of  the  development  of 
this  art  must  be  based  upon  its  chronology.  Therefore  in  the 
first  part  of  this  book  the  attempt  is  made  to  show  the  march  of 
progress  and  to  mention  some  of  the  failures.  The  second  and 
main  portion  of  the  book  is  closely  allied  to  the  historical  divi- 
sion. It  was  necessary  to  make  this  separation  so  as  not  to 
encumber  the  second  part  with  a  tiresome  historical  review.  A 
judicious  selection  was  required  for  the  description  of  the  his- 
torical evolution.  The  purpose  and  scope  of  this  book  does  not 
permit  an  exhaustive  chronological  summary. 

The  main  object  of  this  work  is  treated  in  the  second  part. 
Therefore  only  the  most  important  lines  of  evolution  are  given. 
These  embrace  those  trends  or  influences  which  partly  enter  into 
present  practice  and  partly  on  account  of  the  obvious  failures 
permit  the  young  inventor  to  exercise  his  genius  for  the  discov- 
ery of  more  useful  and  promising  methods. 

In  preparing  this  book  I  have  been  about  equally  author  and 
compiler,  since  I  have  extracted  quite  half  of  its  contents  from 
the  works  of  the  best  writers  and  metallurgists.  Perhaps  it 
would  have  been  better  and  more  acceptable  if  I  had  extracted 
more  and  written  less. 

Still,  possibly  half  is  my  own,  and  in  incorporating  here  the 
thoughts  and  words  of  others  I  have  continually  changed  and 
added  to  their  language,  often  intermingling  in  the  same  sen- 
tence my  own  words  with  theirs.  This  book  being  intended  for 
the  technical  fraternity,  I  have  felt  at  liberty  to  make  from  all 
sources  a  Compendium  of  Coal  Washing;  to  remold  sentences; 
to  change  the  words  and  phrases,  combining  them  with  my 
own  and  using  them  as  if  they  were  my  own,  to  be  dealt  with 
at  my  pleasure  and  so  employed  as  to  make  the  complete  work 
most  valuable  for  the  purpose  intended. 

I  claim,  therefore,  little  merit  of  authorship  and  have  not 


PREFACE  ix 

cared  to  distinguish  my  own  language  from  that  which  I  have 
taken  from  other  sources,  being  quite  willing  that  every  portion 
of  this  book,  in  turn,  may'  be  regarded  as  borrowed  from  some 
more  skilful  author. 

Finally,  it  may  be  said  that  this  book  is  at  present  the  only 
work  which  deals  with  the  subject  of  coal  washing  exclusively. 
And  while  this  volume  does  not  do  this  exhaustively  and  does 
not  claim  to  do  so,  it  is  hoped  that  it  presents  in  a  convenient 
form  the  information  sought  by  coal  operators,  students  and  all 
those  who  are  interested  in  the  art  of  washing  coal. 

In  many  places  in  these  pages  I  have  tried  to  give  credit  to 
the  many  friends  who  have  rendered  me  assistance  in  divers 
ways.  Especially  I  am  under  obligation  to  the  director  of  the 
Bureau  of  Mines  and  the  director  of  the  Engineering  Experi- 
ment Station  of  the  University  of  Illinois  for  their  permission 
to  reprint  part  of  their  publications.  It  only  remains  to  thank 
them  as  a  whole  for  aiding  in  this  work  which  has  been  accom- 
plished in  the  intervals  of  what  I  trust  is  not  otherwise  an  en- 
tirely idle  and  useless  life. 

ERNST  PROCHASKA,  M.E. 
Bonne  Terre,  Missouri, 
November,  1920. 


CONTENTS 


PREFACE vii 

PART  I 

CHAPTER  PAGE 

I  THE  PURPOSE  AND  THE  VALUE  OP  COAL  WASHING     .  1 

II  DEVELOPMENT  OF  THE  PREPARATION  OF  COAL     ...  5 

III  CRUSHING 11 

IV  DEVELOPMENT  AND  THEORY  OF  WET  SEPARATION    .      .  15 
V  THE  EVOLUTION  OF  THE  JIG 31 

VI    OTHER  METHODS  OF  WASHING  COAL 46 

VII  DEWATERING  AND  DRYING  OF  WASHED  COAL      ...  60 

VIII    WATER  CLARIFICATION 64 

IX  TREATMENT  OF   SLUDGE      .     .     .     .     .     .     .    ,.     •  68 

X  CONCLUSION    .     .-';'"£     .......     .     .     .  74 

PART  II 
XI    PROCEEDINGS  AT  THE  MINE      ...     .     ...     .     75 

XII    INTERMEDIATE  UNITS  BETWEEN  THE  SCREENING  PLANT 

AND  THE  WASHERY     .     .     .    '.   '.  .;.     .     .     .     .     85 

XIII  CLASSIFYING  OF  THE  FINE  COAL    .......  103 

XIV  THE  REMOVAL  OF  TRAMP  IRON .  123 

XV    WEIGHING  AND  SAMPLING  APPARATUS     .     .     .  '  .     .  127 

XVI  PREPARATORY  INVESTIGATIONS  .     .     .     ......     .     .  133 

XVII  DIFFERENT  METHODS  OF  WASHING  COAL     ....  157 

XVIII     THE  FEEDING  OF  THE  JIGS  .      . 160 

XIX    TYPES  OF  JIGS 161 

XX     GENERAL  DATA  ON  JIGS 206 

XXI     CONSTRUCTION  OF  JIGS 212 

XXII     CONCENTRATING  TABLES 215 

XXIII    FURTHER  TREATMENT  OF  THE  JIG  PRODUCTS  ....  235 

xi 


Xll 


CONTENTS 


CHAPTER 

XXIV  SUBSEQUENT  TREATMENT  OF  WASHED  NUT  COAL  .     .  236 

XXV    THE  STORAGE  OF  WASHED  NUT  COAL 240 

XXVI    THE  CRUSHING  OF  COAL     .     .     . 241 

XXVII  CRUSHING  AND  RE-WASHING  OF  THE  MIDDLE  PRODUCTS  262 

XXVIII  DEWATERING  AND  STORAGE  OF  FINE  COAL     .     .     .     .  265 

XXIX  WATER  CLARIFICATION  AND  SLUDGE  RECOVERY  .      .      .  286 

XXX     SUBSEQUENT  TREATMENT  OF  SLUDGE 302 

XXXI    PYRITE  RECOVERY     ......' 320 

XXXII    WATER  SYSTEM 339 

XXXIII  POWER 342 

XXXIV  ARRANGEMENT  OF  MOTORS  AND  DRIVES 344 

XXXV    BUILDINGS  AND  STRUCTURES     .     . 348 

XXXVI  COST  OF  WASHING  COAL  ....     .     .     .     .     .     .354 

XXXVII  GENERAL  ARRANGEMENT  OF  WASHERIES  AND  GRAPHI- 
CAL ILLUSTRATIONS  OF  THE  PROCESS    .     .                 .  359 


INDEX 


377 


COAL  WASHING 

PART  I 
THE  DEVELOPMENT  OF  COAL  WASHING 

CHAPTER  I 
THE  PURPOSE  AND  THE  VALUE  OF  WASHING  COAL 

The  existence  of  every  coal  mine  depends  solely  upon  the 
selling  price  of  its  product  exceeding  the  cost  of  production.  It 
may  be  feasible  to  operate  a  washery  or  a  screening  plant  in  con- 
nection with  a  coal  mine  if  the  better  price  obtained  for  the 
cleaner  coal  equals  the  cost  of  operation ;  or  these  plants  may  be 
operated  at  a  loss,  if  thereby  part  of  the  mine  output  can  be  made 
salable  or  its  sales  increased. 

Under  the  last  named  conditions,  however,  the  preparation 
plant  is  only  a  necessary  evil  installed  to  satisfy  the  consumers 
The  management  will  be  inclined  therefore  to  make  this  plant 
as  small  and  as  simple  as  possible.  If,  however,  the  difference 
between  the  selling  prices  of  raw  coal  and  prepared  coal  is  greater 
than  the  cost  of  preparation,  the  preparation  equipment  is  co- 
operating with  the  mine  to  get  a  bigger  profit  for  the  whole 
mining  plant;  and  the  management  will  energetically  use  every 
effort  to  improve  the  method  of  preparation,  to  increase  the  dif- 
ference between  cost  and  selling  price  by  decreasing  the  cost  of 
preparation  and  by  producing  a  high-grade  prepared  coal. 

The  foregoing  facts  are  now  well  known,  but  it  took  a  long 
time  before  the  coal  industry  was  fully  convinced  of  their  truth. 
The  first  preparation  plants  were  not  installed  for  the  purpose 
of  increasing  the  profits  of  a  mine  but  were  forced  upon  the  coal 
industry  by  the  demands  of  the  consumers  for  a  better  product. 

1 


WASHING 


The  first  demaiids  f  0£  cleaner  .coal  were  made  by  the  blast-furnace 
men  who  wanted  a  coke  with  less  ash.  This  demand  for  a  cleaner 
coal  was  restricted  to  comparatively  few  localities,  and  it  was 
only  after  it  had  been  thoroughly  demonstrated  that  the  greatest 
benefit  accruing  from  coal  preparation  lay  in  the  possibility  of 
increasing  the  value  of  the  coal  and  thereby  the  selling  price  far 
above  the  cost  of  preparation,  that  the  coal  industry  took  hold  of 
this  problem. 

Coal  operators  in  the  beginning  felt  little  interest  in  this 
problem  of  producing  cleaner  coal  and  only  those  mines  shipping 
coal  to  blast  furnaces  and  railroads  took  any  interest  in  it  at  all. 
The  reason  for  this  attitude  can  be  found  in  the  view  taken  in 
regard  to  the  purpose  of  coal  washing,  which  a  French  engineer 
"Marsilly"  expressed  as  follows:  "The  purpose  of  coal  wash- 
ing is:  to  get  a  price  for  the  washed  coal  equal  to  the  market 
price  of  the  raw  coal  plus  the  cost  of  washing."  Any  other  pos- 
sible motive  was  at  that  time  not  taken  into  consideration. 

It  will  be  readily  appreciated  that  such  an  opinion  did  not 
encourage  a  vigorous  development  of  coal  washing.  The  ruling 
aim  was,  not  to  decrease  the  economical  results,  obtained  in  the 
past  without  washing,  by  the  introduction  of  this  process.  The 
same  thoughts  can  be  noticed  in  all  publications  during  the  first 
decades  of  coal  washing. 

Not  until  the  year  1864  did  Gaetzschmann  break  through  this 
narrow  view.  He  stated  that  the  main  value  of  coal  washing  lies 
in  the  increase  of  the  selling  price  over  and  above  the  cost  of 
washing.  This  progressive  view  however  did  not  find  general 
recognition.  A  conservative  attitude  on  the  part  of  the  industry 
condemned  his  views  for  almost  20  years. 

In  the  year  1865  Fleck  declares  again,  that  the  cost  of  wash- 
ing should  not  bring  about  an  economical  disadvantage.  The 
lack  of  interest  shown  by  the  coal  operators  in  coal  washing 
forces  Pernolet  in  the  year  1873  to  complain  that  the  purchaser 
is  compelled  to  take  the  screenings  as  they  come  from  the  mine. 
An  almost  complete  resignation  to  existing  regime  is  expressed 
in  the  following :  If  the  coal  mining  company  is  not  at  the  same 
time  interested  in  coke  making,  it  does  not  happen  often  that 
screenings  are  suitably  adapted  for  making  coke.  The  coke 
maker  must  therefore  take  what  the  market  rejects. 


THE  DEVELOPMENT  OF  COAL  WASHING  3 

In  a  similar  manner  Nonne  in  1876  reviews  the  conditions  in 
Westphalia.  With  the  exception  of  the  mines  which  are  forced 
by  extremely  dirty  coal  or  by  special  market  conditions,  to  wash 
the  coal,  all  other  mines  have  always  considered  a  washer  as  an 
expensive  and  troublesome  appendix  which  should  be  avoided  by 
all  means.  Nonne  replied  to  this  by  proving  that  the  value  of 
coal  increases  faster  than  the  ash  contents  decrease.  At  this  time 
his  warning  was  very  appropriate,  because  on  account  of  the 
existing  boom  times,  the  best  coal  was  frequently,  either  purposely 
or  by  accident,  mixed  by  the  miners  as  well  as  the  loaders  with 
slate,  rock  and  bugdust,  because  being  pushed  from  all  sides, 
there  wras  no  time  for  a  proper  preparation. 

Besides  the  visible  advantage  of  a  larger  profit,  the  economic 
question  of  the  fuel  supply  commenced  to  be  understood.  It 
was  plainly  visible  that  a  properly  functioning  separation  plant 
could  permit  the  mining  of  a  coal  bed,  the  coal  from  which  on 
account  of  its  large  percentage  of  impurities  could  not  be  sold  in 
the  raw  state. 

Only  after  a  full  realization  of  the  truth  of  the  above  state- 
ment did  the  process  of  coal  washing  enter  upon  that  period 
of  development  which  took  into  account  the  importance  of  this 
process  and  permitted  the  utilization  of  all  its  advantages  to  the 
fullest  extent. 

Even  as  late  as  in  the  year  1918  Thomas  James  Drakeley  states 
that  washing  is  regarded  as  a  troublesome  and  expensive  opera- 
tion at  most  collieries  and  that  only  once  in  his  experience  was 
unqualified  satisfaction  expressed  with  regard  to  a  washer. 

The  endeavors  of  a  good  many  inventors  however  outstripped 
the  practical  views  of  the  industry  and  in  some  instances  over- 
shot their  scope.  Enthusiastic  inventors  full  of  hope  forgot,  in 
their  eagerness  to  clean  the  coal,  the  cost  of  performing  this 
operation  and  also  the  necessary  simplicity  and  efficiency  of  the 
apparatus  to  be  employed.  Consequently  a  good  many  devices 
appeared  which  were  sufficiently  complicated  for  the  concentra- 
tion of  even  complex  ores,  but  which  were  neither  useful  nor 
necessary  for  the  washing  of  coal. 

There  were  plenty  of  failures  and  it  must  in  justice  be  said 
that  such  failures  have  been  responsible  in  many  cases  for  the 
unfavorable  opinion  in  regard  to  the  washing  of  coal.  A  good 


4  COAL  WASHING 

many  coal  washers  built  as  late  as  the  seventies  of  the  last  cen- 
tury were  abandoned  and  dismantled. 

After  about  70  years  of  evolution  however  a  fairly  clear 
appreciation  of  the  advantages  and  limitations  of  this  method 
of  coal  preparation  has  been  reached.  After  the  resistance  of 
operators  against  washing  had  been  finally  overcome  and  their 
first  eagerness  modified,  practical  experience  came  forward  as  a 
splendid  teacher.  This  experience  is  at  present  sufficiently  de- 
veloped to  confute  any  dogmatic  unfavorable  opinion  in  regard 
to  coal  washing  and  to  establish  the  following  maxims: 

1.  The  preparation  of  coal  shall,  by  the  cleaning  of  the  raw  material 
and  the  production  of  suitable  and  well  screened  sizes,  secure  a  maximum 
price  per  ton  of  output. 

2.  To  arrive  at  this  result  three  points  must  be  kept  in  view:      (a)  High- 
est possible  purity  of  coal;    (b)    smallest  possible  loss  of  coal;    (c)    small 
cost  of  production. 

3.  As  the  foregoing  three  demands  are  conflicting,  it  will  be  necessary 
for  the  proper  and  economical  installation  of  a  preparation  plant  to  find  in 
each  case  the  best  relation  between  the  three  factors. 


CHAPTER  II 
•  DEVELOPMENT  OF  THE  PREPARATION  OF  COAL 

General.  Only  by  consulting  the  technical  journals  is  it  pos- 
sible to  follow  the  development  of  the  preparation  of  coal  to  its 
very  beginning.  Any  description  of  this  development  neces- 
sarily will  be  incomplete  as  not  all  experiments  found  their  way 
into  print.  During  the  earlier  periods  means  of  communication 
were  not  plentiful  and  authors  were  compelled  to  restrict  their 
investigations  within  quite  limited  zones.  This  also  explains  the 
fact  that  the  development  of  coal  preparation  showed  at  corre- 
sponding periods  a  very  different  degree  of  advancement  in 
different  localities.  While  in  one  district  the  mechanically  op- 
erated and  continuously  working  jig  was  the  accepted  apparatus, 
other  districts  were  still  using  hand  actuated  jigs.  Crude  screen- 
ing plants  with  wicker  work  screening  surfaces  are  contemporary 
with  conical  revolving  screens  with  mantles  made  of  perforated 
iron  plates. 

Only  from  the  time  when  international  industrial  exhibitions 
were  held  in  different  countries  onward  can  a  uniform  develop- 
ment of  methods  and  apparatus  be  noticed.  The  exhibitions  also 
brought  about  a  quicker  advancement  on  account  of  the  avoid- 
ance of  mistakes,  which  were  quickly  recognized  through  the  ex- 
change of  views  between  the  different  districts. 

The  same  division  will  be  used  in  the  description  of  the  his- 
torical development  as  is  employed  in  the  second  portion  of  this 
book,  so  as  to  facilitate  the  reference  from  one  part  to  the  other. 

Methods  used  in  the  Mine.  The  maxim  of  loading  the  coal  as 
clean  as  possible  has  been  known  and  practiced  to  a  greater  or 
less  extent  since  coal  has  been  mined.  As  early  as  the  first  quar- 
ter of  the  last  century  slate  was  picked  out  of  the  coal  in  the 
mine  and  gobbed.  But  in  the  earlier  periods  this  practice  of 
hoisting  only  clean  coal  was  carried  still  farther  and  the  coal 
operators  tried,  following  the  example  given  by  the  ore  miners, 

5 


6  COAL  WASHING 

to  carry  on  the  preparation  of  coal  within  the  mine  itself.  The 
most  widely  used  method  was  that  of  loading  only  lump  coal  by 
using  forks  with  tines  spaced  at  a  certain  interval.  To  separate 
the  coal  into  different  sizes  was  also  attempted.  This  was  car- 
ried on  with  especial  care  in  some  mines  in  Saxony. 

After  the  coal  was  shot  down,  the  floor  of  the  room  was  care- 
fully brushed  off  and  the  screened  coal  deposited  on  the  floor  in 
three  or  four  heaps,  according  to  their  respective  sizes.  Such 
methods,  however,  could  not  prevail  for  long.  With  the  increase 
in  tonnage  produced  and  the  advance  in  labor  cost,  the  amount 
of  coal  mined  per  man  was  of  greater  influence  than  the  quality 
of  the  coal.  The  system  of  paying  the  miner  only  for  the  lump 
coal  could  only  be  applied  at  mines  where  the  market  price  for 
lump  was  much  higher  than  that  for  mine-run. 

Generally  speaking,  a  strict  inspection  combined  with  a  just 
docking  system  and  the  avoidance  of  breaking  the  coal  on  its 
way  through  the  tipple,  are  the  only  means  employed  at  the  pres- 
ent time  to  secure  the  loading  of  a  clean,  well  sized  product. 

Dry  Separation.  Dry  separation  is  usually  accomplished  with 
screens  and  the  methods  of  screening  can  be  separated  into  two 
groups:  The  first  deals  with  screens  for  the  purpose  of  making 
lump  coal  and  screenings  only;  the  second  deals  with  screens 
that  classify  the  product  into  lump,  egg,  nut  and  different  sizes 
of  screenings.  The  first  group  belongs  solely  to  the  dry  separa- 
tion process,  whereas  the  second  group  belongs  to  it  only  if  all 
the  coal  is  prepared  in  a  dry  state.  If  a  washery  is  connected 
with  the  mine  the  classification  screens  belong  properly  to  the 
washing  plant. 

Mechanical  Preparation  of  the  Screened  Coal.  At  present 
the  preparation  of  the  screenings  is  carried  on  with  water  and  is 
mostly  done  in  jigs.  In  the  beginning,  however,  many  processes 
were  devised  to  avoid  the  use  of  water  and  even  now  many  in- 
ventors are  working  on  the  solution  of  other  separation  proc- 
esses, which  will  take  the  place  of  wet  separation.  New  devices 
and  apparatus  are  constantly  appearing  and  it  seems  therefore 
judicious  to  enter  more  in  detail  into  the  past  history.  By  this 
means  many 'ideas  which  at  present  are  considered  novel  will  be 
found  to  be  only  returns  to  a  circuitous  path  of  development. 
The  wet  separation  always  has  had  a  decided  advantage.  Be- 


THE  DEVELOPMENT  OF  COAL  WASHING  1 

fore  we  enter  into  the  description  of  this  process,  however,  we 
must  consider  the  development  of  the  classification  of  fine  coal. 
Two  different  opinions  in  regard  to  the  proper  way  of  carrying 
on  screening  have  always  existed:  (1)  Screening  from  coarse 
to  fine  and  (2)  screening  from  fine  to  coarse.  There  also  exists 
the  question  of  whether  it  is  better  to  screen  before  washing  or 
to  wash  first  and  then  screen. 

In  the  beginning  screening  from  fine  to  coarse  was  preferred. 
It  was  the  shortest  method  and  had  the  advantage  that  the  dust 
was  removed  on  the  first  screen  plate,  or  that  having  the  smallest 
perforations.  But  it  had  the  drawback  that  the  fine  screens  wore 
out  quickly  on  account  of  the  large  mass  of  coal  passing  over 
them,  and  furthermore  this  large  mass  of  big  coal  did  not  permit 
perfect  screening.  Both  these  disadvantages  became  greater  as 
the  quantities  to  be  handled  increased.  In  the  sixties  of  the  last 
century  the  practice  reversed  itself  completely  and  most  screen- 
ing was  done  with  concentric  revolving  screens.  This  method, 
however,  did  not  remain  long  in  actual  practice.  The  demand 
was  for  a  method  of  screening  that  would  give  output,  durability 
and  simplicity  combined.  These  three  demands  were  fulfilled  by 
the  shaking  screen  and  later  by  the  vibrating  screen. 

The  importance  attaching  to  the  order  in  which  the  different 
sizes  are  screened  off  slowly  disappeared  and  in  its  place  arose 
the  following  questions :  Which  arrangement  of  screening  appa- 
ratus will  best  fit  into  the  general  layout  and  permit  the  best 
possible  use  of  the  space  at  disposal?  Furthermore,  which  will 
permit  the  simplest  and  most  economical  conveying  of  the  differ- 
ent sizes  to  the  next  following  piece  of  apparatus?  More  or  less 
freedom  in  all  dimensions  of  a  contemplated  screening  plant  is 
at  present  the  deciding  factor  and  as  the  different  types  of 
screens  vary  much  in  their  dimensions,  we  encounter  not  only  the 
most  extreme  types  but  also  a  great  many  variations  combining 
different  types  in  one  plant.  Of  course,  if  sufficient  room  is 
available,  the  screening  plant  will  be  adapted  in  each  separate 
case  to  the  physical  condition  of  the  coal.  Important  for  the 
present  consideration  is  the  fact  that  the  development  of  practi- 
cal experience  has  robbed  the  question  of  coal  classification  pre- 
paratory to  washing  of  its  fundamental  importance. 

As  long  as  jigging  was  performed  on  hand  jigs  nobody  thought 


8  COAL  WASHING 

about  classifying  the  coal.  This,  however,  was  not  because  the 
operator  was  convinced  of  the  correctness  of  his  method,  but 
because  at  that  time  only  the  simplest  and  cheapest  pieces  of 
apparatus  were  used.  Later  on,  after  the  jigs  were  improved,  it 
was  found  that  the  separation  was  not  particularly  close  and 
lack  of  sizing  was  blamed  therefor.  Since  in  ore  dressing  plants 
close  classification  was  considered  necessary,  this  was  followed 
also  in  the  coal  washeries. 

At  first  this  was  carried  too  far,  and  even  the  fine  coal  was 
divided  into  three  or  four  sizes.  But  later  on  it  was  found  that 
this  close  sizing  produced  too  tight  a  bed  in  the  jigs  and  it  be- 
came necessary  to  mix  in  some  nut  coal.  Nevertheless  sizing  ap- 
paratus remained  a  part  of  the  equipment  of  every  washery. 
The  screening  of  the  coal,  however,  was  troublesome  and  made 
the  washery  complicated. 

Close  classification  (except  in  the  case  of  very  small  sizes) 
was  in  use  until  the  end  of  the  seventies. .  It  was  not,  however, 
proved  to  be  correct  either  by  theoretical  investigations  or  by 
practical  comparative  tests.  In  the  beginning  of  the  eighties 
the  opinion  in  regard  to  classification  changed  slowly.  It  was 
remembered,  that  it  is  possible  to  separate  in  a  jig,  materials 
containing  grains  of  different  diameter.  It  was  also  discovered 
that  the  specific  gravity  of  the  materials  encountered  was  favor- 
able for  jigging.  In  the  year  1878,  when  Althans  published  his 
review  of  the  status  of  coal  washing,  jigs  for  classified  and  un- 
classified coal  were  used  indiscriminately  side  by  side. 

Every  effort  was  now  made  in  classifying  plants  to  simplify 
the  total  arrangement  of  the  coal  washery.  Althans  describes 
the  prevailing  views  in  regard  to  this  as  follows :  If  previously 
the  theoretical  efforts  towards  an  improvement  in  coal  washing 
were  carried  too  far  and  resulted  in  the  arrangement  of  compli- 
cated installations,  we  find  in  the  newer  plants,  even,  where  the 
most  perfect  apparatus  is  used,  preference  shown  to  compara- 
tively simple  methods.  Artois  also  notes  that  the  compromise  in 
regard  to  classification  of  coal  to  be  washed  remained  in  vogue 
for  several  years.  Later  on,  however,  more  attention  was  paid 
to  a  minute  classification  before  washing.  A  great  number  of 
washeries  which  were  described  in  the  technical  journals  of  that 
period  are  proof  of  the  changed  views,  QIJ  this,  subject.  A  return 


THE  DEVELOPMENT  OF  COAL  WASHING  9 

to  the  former  views  was  partly  brought  about  because  the  rea- 
sons given  for  the  other  methods  were  not  properly  or  at  least  not 
exhaustively  stated.  It  was  a  great  mistake  to  use  these  views 
as  a  guide  in  all  cases. 

In  1890  Remy  points  out  that  the  nature  of  the  raw  coal  must 
be  considered  in  each  separate  case  in  order  to  decide  intelli- 
gently on  the  proper  type  of  classification  method.  Even  at 
present  we  have  no  absolute  convincing  answer  to  the  problem 
of  the  best  procedure  to  follow. 

Kemy's  statement  describes  the  situation  correctly.  Both  sys- 
tems were  used  side  by  side,  but  the  classification  before  washing 
had  the  preference.  There  was  little  reason  to  change  this 
method,  because  the  market  demanded  a  great  number  of  differ- 
ent sizes.  It  is,  however,  of  interest  to  note  that  this  demand 
caused  the  first  break  in  the  method  of  classifying  before  wash- 
ing. The  sharper  competition  and  the  increased  demands  of  the 
consumers  called  for  exact  screening  and  with  the  beginning  of 
this  century  it  was  found  advisable  to  limit  the  classification 
before  washing,  because  a  classification  after  washing  prevented 
considerable  abrasion  and  permitted  the  loading  of  a  perfectly 
screened  coal. 

In  adherence  to  the  previous  methods,  a  classification  into  two 
or  three  sizes  was  still  prevalent.  All  the  washed  coal,  how- 
ever, was  re-united  after  washing  and  finally  screened  into  the 
market  grades.  The  latest  step  was  taken  by  Baum  in  Europe 
and  Stewart  in  America,  who  used  the  slogan :  First  washing — 
then  screening.  Large  jigs  were  introduced,  taking  all  screen- 
ings from  3  in.  down.  But  even  this  simple  method  is  again  sur- 
passed by  the  more  modern  system  of  using  separate  jigs  or  con- 
centrating tables  for  coal  smaller  than  %6  or  %  in.  This  method 
is,  however,  only  in  its  infancy  and  a  great  majority  of  American 
washeries  are  handling  unscreened  coal  from  3  in.  down  or  if 
the  coal  is  to  be  used  for  coke  making  from  1  in.  down. 

The  choice  of  the  proper  method  to  be  pursued  in  coal  wash- 
ing is  an  extremely  difficult  matter.  It  frequently  happens  that 
a  method  effective  at  one  mine  will  not  produce  the  desired  re- 
sults if  it  is  adopted  in  its  entirety  at  another.  At  each  mine 
there  are  peculiarities  which  demand  special  treatment. 

The   deciding   factor  in  the   selection  of  a   suitable   washer 


10  COAL  WASHING 

should  be  the  physical  and  chemical  characteristics  of  the  raw 
coal.  It  is  only  by  expending  much  thought  on  the  problem  of 
selection  that  disappointments  may  be  prevented. 

Coal  washing  is  rapidly  coming  into  its  own  and  before  many 
years  as  careful  attention  will  be  given  to  the  design  and  con- 
struction of  coal  washeries  as  is  now  being  expended  upon  by- 
product plants  and  other  allied  industries.  Heretofore,  coal 
washing  has  been  considered  only  as  an  incidental  and  a  neces- 
sary evil  instead  of  the  big  problem  it  really  is — one  worthy  of 
the  most  careful  study  by  men  well  trained  along  technical  lines 
and  capable  of  delving  into  the  fundamental  principles. 

The  knowledge  gained  by  past  experience  can  be  condensed 
into  the  following  paragraphs : 

The  jigging  of  bituminous  coal  can  be  carried  on  successfully 
without  preparatory  sizing. 

Experience  has  shown  that  the  separate  washing  of  the  fines 
facilitates  the  washing  process  and  gives  slightly  better  results. 
It  has  been  proved,  however,  that  coal  finer  than  10  mesh  can  not 
be  improved  by  washing  and  should,  if  low  enough  in  ash,  be 
mixed  with  the  washed  coal ;  or  if  too  high  in  ash  used  for  other 
purposes. 

The  advisability  of  sizing  the  larger  pieces  of  coal  before 
washing  must  be  judged  for  each  installation  separately. 

To  produce  a  perfectly  sized  washed  coal  screening  after  wash- 
ing is  necessary. 

The  whole  question,  where  favorable  raw  coal  is  involved,  is 
not  of  decided  importance.  It  must  be  determined  in  most  cases 
by  considering  the  cost  of  installation  and  operation,  the  space 
required  and  at  disposal,  and  the  most  suitable  general  arrange- 
ment of  the  plant. 


CHAPTER  III 
CRUSHING 

General.  A  crushing  plant  must  in  many  cases  be  added  to 
the  preparatory  apparatus  of  a  coal  washery.  The  crushing  is 
either  for  the  purpose  of  separating  the  pure  coal  from  the  slate 
and  pyrites,  or  producing  a  coal  of  suitable  size. 

The  proper  design  of  the  first  crushing  plants  suffered,  as  did 
all  the  other  units  of  a  coal  washery,  through  the  lack  of  proper 
research  studies  carried  on  by  technically  trained  men.  The 
fact  that  coal  should  be  crushed  without  producing  too  much 
fines  and  dust  was  known  in  the  year  1840  by  the  anthracite 
coal  operators  of  Pennsylvania.  Berard  transmitted  this  knowl- 
edge to  Europe  in  1850.  But  that  all  precautions  must  be  taken 
to  restrict  the  crushing  to  its  proper  sphere  was  only  recognized 
in  the  eighties,  when  attention  was  paid  to  the  handling  of  mid- 
dle products. 

In  the  case  of  coking  coal  it  was  always  the  prevailing  idea  to 
crush  as  fine  as  possible.  This  did  not  take  into  consideration 
that  thereby  the  slate  is  pulverized  to  the  same  degree  as  the 
coal,  which  causes  trouble  in  the  jigs  and  makes  the  washing 
difficult.  The  following  statements  show  how  little  proper  crush- 
ing was  understood  at  that  time. 

Meynier  in  1857  says:  "If  coal  of  different  sizes  must  be 
washed  it  is  advisable  to  disintegrate  it  in  order  to  facilitate  the 
washing,  especially  if  the  coal  contains  laminated  flakes  of 
pyrites  and  slate."  Further  on  he  states:  "To  separate  the 
slate  it  is  advisable  that  the  coal  is  reduced  to  fine  dust.  As  the 
slate  is  mostly  attached  to  the  coal  in  thin  slivers,  it  breaks  off 
by  crushing  and  can  be  separated  without  great  loss." 

To  the  contrary  Gaetzschmann  in  his  textbook  published  in 
1864  says : 

"The  crushing  of  coal  is  only  advisable  in  certain,  rare  cases,  and  there 
is  no  reason  for  the  crushing  plan  to  be  an  essential  part  of  every  coal 

11 


12  COAL  WASHING 

washery.  A  suitable  classification  will  make  the  crushing  rolls  super- 
fluous. The  crushing  of  coal  has  not  always,  as  is  sometimes  supposed, 
the  effect  of  separating  the  slate  from  the  coal,  because  with  some  coal  the 
adhesion  of  slate  to  the  coal  is  greater  than  that  of  the  coal  particles  to 
each  other." 

David  Hancock  in  one  of  his  reports  on  a  contemplated  washery  says: 
"If  you  could  crush  the  coal  without  crushing  the  impurities,  I  would  say 
that  a  size  smaller  than  f  in.  would  be  profitable,  but  the  trouble  is  that 
you  are  going  to  crush  the  slate  as  well,  and  instead  of  having  8.4  per 
cent,  of  stuff  finer  than  20  mesh  running  10.90  per  cent,  ash,  you  are  going 
to  get  about  16  per  cent,  running  in  ash  up  to  or  near  the  raw  coal  or 
about  18.0  per  cent. 

The  almost  general  use  of  crushers  in  Gaetzschmann  's  time 
can  be  explained  by  the  fact  that  at  that  period  the  major  por- 
tion of  the  fine  coal  was  lost  and  wasted  on  the  refuse  dump. 
This  loss  of  fine  coal  reduced  the  amount  of  a  size  needed  for 
coke  making  to  such  an  extent  that  some  of  the  coarser  coal  had 
to  be  crushed. 

By  using  better  methods  of  washing  the  finer  coals,  this  extra 
supply  of  fine  material  was  no  longer  needed  and  it  was  possible 
to  restrict  the  crushing.  In  the  eighties  a  start  was  made  to 
separate  the  slate  disseminated  throughout  the  coal  on  a  picking 
belt  or  in  jigs  and  to  crush  only  the  middle  product  containing 
slate  intimately  mixed  with  the  coal. 

After  cognizance  had  been  taken  of  the  bad  results  obtained 
from  crushing  before  washing,  the  methods  employed  developed 
in  such  a  way  that  coal  was  crushed  after  washing  and  the  crush- 
ing was  restricted  to  the  middle  products  only.  At  present  we 
can  adopt  for  this  method  the  slogan:  "First  Washing — Then 
Crushing." 

Some  washeries  go  so  far  at  present  as  to  rewash  the  middle 
products,  without  crushing,  for  the  purpose  of  separating  the 
pure  slate  and  to  prevent  it  from  being  crushed.  The  descrip- 
tion of  the  difficulties  encountered  in  the  clarification  of  the 
wash  water  will  show  that  there  is  some  well-founded  reason  for 
using  such  a  method. 

During  the  last  ten  years  more  attention  has  been  paid  to  the 
crushing  of  lump  coal  into  smaller  sizes.  At  least  one  mine  in 
Illinois  is  crushing  the  whole  output.  The  general  use  of  me- 
chanical stokers  and  the  manufacture  of  gas  calls  for  nut  coal 


THE  DEVELOPMENT  OF  COAL  WASHING 


13 


and  screenings,  so  that  in  some  districts  the  price  of  nut  coal 
has  advanced  over  that  of  lump. 

The  modern  requirements  for  a  crushing  plant  are  as  follows: 

1.  Crushing  lump  coal  to  nut,  avoiding  thereby  as  much  as  possible  the 

making  of  fines  and  dust. 

2.  Crushing  the  middle  products,  for  the  purpose  of  separating  the  adher- 

ing slate  and  pyrite. 

3.  Crushing  and  mixing  different  coals  to  be  used  for  coke  making. 

Crushing-  Apparatus.     As  early  as  1840  roll  crushers  were 
used  in  Pennsylvania  to  crush  lump  coal  down  to  nut.     Berard 


Fig.  1.    Different  Types  of  Rolls  used  for  Crushing  Coal 

introduced  this  type  of  crusher  into  Europe,  where  it  is  still  in 
use,  together  with  gyratory  crushers  while  even  pug  and  ball 
mills  have  been  used.  In  the  year  1870  Carr  introduced  in 
England  a  disintegrator  similar  in  design  to  the  Stedman  dis- 
integrator used  in  America.  Jaw  crushers  for  coal  were  used 
in  the  eighties  in  Europe  and  at  present  a  great  variety  of 
crushers  are  there  employed.  Jaw  crushers  were  modified  into 
needle  crushers,  to  prevent  the  making  of  too  much  fines. 

Roll  crushers  have  either  smooth,  corrugated  or  toothed  rolls. 
Disintegrators  are  mostly  used  to  pulverize  and  mix  coal  to  be 


14 


COAL  WASH IX G 


used  for  coke  making.  In  America  roll  crushers  with  either 
corrugated  or  toothed  rolls  are  the  accepted  standard  type. 
Toothed  rolls  are  used  for  the  preliminary  breaking  of  the  lump 
and  the  corrugated  rolls  for  the  final  crushing.  Fig.  1  shows  a 
variety  of  rolls  and  Fig.  2  gives  a  good  illustration  of  a  large 


Fig.  2.    Belt-driven  Coal  Crusher 

belt  driven  crusher,  having  a  maximum  capacity  of  1,000  tons 
of  coal  per  hour. 

In  selecting  a  crusher  it  is  advisable  in  each  separate  case 
to  choose  a  machine  adapted  to  the  characteristics  of  the  coal 
and  to  combine  the  quantitative  and  qualitative  capacity  so  far 
as  possible  with  a  simple  and  rugged  construction. 


CHAPTER  IV 
DEVELOPMENT  AND  THEORY  OF  WET  SEPARATION 

Wet  separation  has  been  and  is  still  the  fundamental  opera- 
tion in  coal  washing  and  the  genius  of  inventors  has  been  princi- 
pally exercised  to  improve  this  process.  The  theoretical  princi- 
ples of  "wet  mechanical  separation"  were  well  developed  and 
were  followed  to  a  great  extent  in  ore  dressing,  at  the  time  the 
first  timid  experiments  were  made  in  coal  washing.  Conse- 
quently coal  washing  followed  in  the  footsteps  of  ore  dressing 
and  the  following  theory  of  jigging,  even  if  dealing  with  metal- 
liferous materials,  can  be  adapted  with  slight  changes  to  coal 
washing. 

The  Process  of  Jigging.  Jigs  are  usually  built  in  the  shape 
of  a  tank,  divided  into  two  compartments  by  a  partition.  This 
partition  is  not  carried  quite  to  the  bottom  of  the  tank,  so  that 
the  two  compartments  are  intercommunicating.  In  one  of  the 
compartments  a  screen  is  securely  fastened,  somewhat  below  the 
top  edge  of  the  tank  in  a  horizontal  or  slightly  inclined  position. 
In  the  other  compartment,  a  plunger  can  be  moved  up  and  down 
by  mechanical  means. 

The  tank  is  filled  with  water  to  such  a  height  that  the  screen 
and  the  superimposed  bed  of  material  to  be  treated  is  always 
submerged. 

The  material  to  be  jigged  is  spread  out  over  the  screen,  while 
the  plunger  is  in  motion.  On  the  down  stroke  of  the  plunger 
the  water  flows  from  the  plunger  compartment  into  the  screen 
compartment  and  through  the  perforations  of  the  screen.  This 
action  lifts  the  bed  off  the  screen.  At  the  up  stroke  of  the 
plunger  the  water  recedes  and  the  bed  drops  back  upon  the 
screen.  By  this  means  the  particles  of  the  bed  become  stratified 
in  such  a  way  that  the  denser  particles  collect  upon  the  screen, 
whereas  the  lighter  ones  will  be  pushed  to  the  top  of  the  bed. 
By  repeating  this  plunger  motion,  a  complete  separation  of  the 

15 


16  COAL  WASHING 

different  particles  according  to  their  specific  gravities  can  be  ac- 
complished, provided  the  difference  in  the  specific  gravities  of 
the  separate  particles  is  well  defined  and  the  jig  is  properly  de- 
signed and  operated. 

This  fact  has  been  known  for  a  long  time  and  applied  in  actual 
practice.  But  no  entirely  satisfactorily  theoretical  explanation 
has  been  given  to  show  clearly  how  and  why  this  separation  takes 
place. 

Linkenbach  takes  the  stand  that  classified  grains,  i.  e.,  mate- 
rials collected  by  screening  into  a  group  or  groups  of  approxi- 
mately ^  the  same  size,  are  separated  in  a  jig  according  to  the 
law  of  equal  settling.  He  says  that  the  specifically  lighter 
grains  are  lifted  higher  than  the  denser  ones  and  that  the  lighter 
particles  do  not  drop  back  quite  so  far  as  the  denser  ones ;  so  that 
by  repeating  the  plunger  motion,  the  distance  between  the  lighter 
and  the  denser  grains  will  become  greater  and  greater.  He  con- 
siders that  the  separation  of  a  screened  product  in  a  jig  is  simi- 
lar to  classification. 

Bilharz  states  that  the  equal  settling  qualities  of  solid  grains 
are  important  means  for  the  separation  of  pieces  of  different 
specific  gravities,  whose  diameter,  however,  are  in  a  certain  pro- 
portion to  one  another,  either  in  a  quiet  body  of  water  or  still 
more  effectively  in  an  ascending  column  of  water.  He  further- 
more states  that  if  the  material  to  be  treated  contains  several 
different  substances  the  separation  takes  place  in  an  intermit- 
tently ascending  column  of  water.  This  upward  movement  of 
the  water  can  be  produced  either  by  moving  the  screen  in  a 
quiet  body  of  water  or  by  moving  the  water  column  by  means 
of  a  plunger;  the  material  resting  in  the  latter  case  on  a  fixed 
screen.  In  both  of  these  cases  the  upward  moving  grains  will 
drop  back  at  each  return  stroke  of  the  plunger  and  eventually 
will  assume  stratified  layers,  according  to  their  specific  gravities, 
the  heavy  ones  on  the  bottom  and  the  light  ones  on  top.  This  is 
the  fundamental  principle  of  the  jigging  process,  which  is  only 
limited  by  the  fineness  of  the  grains  to  be  treated. 

Bilharz   also  believes   that   the   particles   of   material   to   be 
treated  arrange  themselves  during  their  upward  travel  according  - 
to  their  respective  specific  gravities  and  that  they  remain  in  the 
same  order  during  the  following  downward  motion,    This  opin- 


THE  DEVELOPMENT  OF  COAL  WASHING  17 

ion  is  also  shared  by  Professor  Robert  H.  Richards,  who  treats 
this  subject  fully  in  his  book  on  "Ore  Dressing." 

Sparre  comes  to  the  conclusion  that  besides  the  law  of  free 
settling  other  forces  influence  the  work  of  a  jig.  He  believes 
that,  as  in  jig  work,  the  distances  traversed  by  the  grains  are 
quite  small,  the  velocity  at  the  beginning  of  each  stroke  has  some 
influence  on  the  work  performed  and  that  the  different  spaces 
through  which  the  grains  pass  with  an  accelerated  velocity  must 
be  compared  in  order  to  arrive  at  a  correct  understanding  of 
the  jig  work. 

Sparre  computed  a  table,  which  shows  the  starting  velocities 
and  the  spaces  traversed  by  galena  and  quartz  balls  of  different 
diameter,  also  the  resulting  differences  in  the  distances  traversed 
during  fractions  of  one  second. 

The  following  table  is  figured  in  metric  measurements.  The 
specific  gravity  of  galena  is  taken  at  7.5  and  that  of  quartz  -at 
2.5  instead  of  2.6  in  order  to  get  a  simpler  relation  between  the 
diameters.  Two  sizes  of  galena  are  compared  with  two  sizes  of 
quartz  having  the  same  settling  velocity  and  one  very  coarse 
grain  of  quartz  is  considered  in  order  to  show  that  the  smallest 
grain  of  galena  can  be  separated  from  the  coarsest  grain  of 
quartz  on  a  jig,  by  using  only  moderate  pulsion,  not  exceeding 
%  in.  lift  of  the  bed. 

A  comparison  of  the  behavior  of  grains  of  the  same  diameter 
shows  that  the  difference  between  the  respective  distances  in- 
creases steadily.  The  quicker  falling  galena  gets  farther  and 
farther  away  from  the  slower  moving  quartz.  The  increase  in 
the  distance  between  the  two  grains  during  equal  periods  of 
time  grows  steadily,  until  after  a  certain  length  of  time  it  be- 
comes a  constant.  But  if  we  study  the  behavior  of  a  galena 
grain  of  0.5  m/m.  and  its  relation  towards  a  quartz  grain  of 
large  diameter,  we  find  that  the  difference  in  the  traversed 
spaces  decreases  in  proportion  to  the  difference  between  the 
diameters  of  the  grains.  While  the  quartz  grain  of  2.18  m/m. 
diameter  even  after  %o  of  a  second  remains  behind  the  galena 
grain  of  0.5  m/m.  diameter,  the  quartz  grains  of  9.45  m/m.  and 
18.9  m/m.  diameter  advance  after  Vw  of  a  second  over  the  galena 
grain  of  0.5  m/m.  diameter. 

But  even  with  the  above  sized  grains,  the  distances  between  the 


18 


COAL  WASHING 


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THE  DEVELOPMENT  OF  COAL  WASHING  19 

quicker  falling  galena  and  the  slower  falling  quartz  increases 
at  the  start  according  to  the  specific  gravities  of  the  respective 
grains.  However,  this  advance  of  the  denser  grains  is  getting 
smaller  in  indirect  proportion  with  the  difference  in  the  diame- 
ters of  the  grains.  The  galena  grain  of  0.5  m/m.  diameter  has 
even  after  %o  of  a  second  an  advance  of  3.3  m/m.  over  the  quartz 
grain  of  2.18  m/m.  diameter,  but  the  quartz  grains  of  9.45  m/m. 
and  18.9  m/m.  diameter  are  getting  ahead  after  M.O  of  a  second, 
while  after  }&>  of  a  second  the  advance  of  the  galena  grain  of 
0.5  m/m.  diameter  over  the  quartz  grain  of  9.45  m/m.  diameter 
was  still  1  m/m.,  and  over  the  quartz  grain  of  18.9  m/m.  diame- 
ter 0.69  m/m. 

If  we  therefore  restrict  the  total  distance  through  which  the 
grains  are  permitted  to  fall  to  7.88  m/m.,  i.  e.,  the  distance 
reached  by  a  galena  grain  of  0.5  m/m.  diameter  after  ¥20  of  a  sec- 
ond, and  interrupt  the  motion  at  that  point,  the  grains  will  fall 
through  the  same  space  if  allowed  to  settle  again.  Thus  we  see 
that  with  each  repetition  of  the  pulsation  the  distance  between 
the  galena  and  quartz  grains  will  be  increased  the  same  amount, 
namely,  1  m/m.  and  0.69  m/m.,  respectively,  so  that,  to  get  a 
distance  of  10  m/m.,  and  6.9  m/m.  ten  plunger  strokes  will  be 
required. 

Sparre  therefore  comes  to  the  conclusion,  that  by  using  a  cor- 
rect number  and  length  of  plunger  strokes,  it  will  be  possible  to 
make  in  a  jig  a  complete  separation  of  materials  having  different 
specific  gravities. 

The  theoretical  foundation  for  jig  work  lies  in  the  fact  that 
the  materials  to  be  treated  move  over  small  distances  only  with 
an  accelerated  velocity. 

In  jigging,  however,  the  grains  do  not  only  fall  in  an  ascend- 
ing column  of  water,  but  they  receive  also  through  the  sudden 
influx  of  .water  a  certain  impulse,  which  modifies  their  move- 
ments to  the  same  extent  as  the  accelerated  settling  velocities  in 
a  resisting  medium,  or  the  law  of  hindered  settling,  modifies  the 
jigging  process 

Sparre,  however,  believes  that  the  influence  of  the  water  im- 
pulse can  be  neglected,  on  account  of  the  resistance  offered  to 
the  ascending  water  column  by  the  screen  itself  and  by  the 
material  resting  upon  it.  The  irregularities  of  the  interstices 


20  COAL  WASHING 

between  the  particles  of  the  bed  are  such  that  a  vertical  acting 
water  impulse  will  almost  entirely  disappear.  Sparre  considers 
only  the  ascending  water  column  as  the  effective  medium  and 
believes  that  the  impulse  of  the  ascending  column  as  well  as  the 
suction  effect  of  the  descending  column  are  ineffective  or  at  least 
of  little  importance. 

Rittinger  gives  a  somewhat  different  explanation  of  the  proc- 
ess of  jigging.  Guided  by  a  formula  developed  by  him  for  the 
starting  motion  of  solid  bodies  in  quiet,  ascending  and  descend- 
ing water  columns,  he  has  calculated  the  velocities  and  distances 
for  fractions  of  a  second.  In  his  treatise  on  ore  dressing  Rit- 
tinger gives  a  good  many  tables,  showing  the  behavior  of  mate- 
rials of  different  sizes  and  different  specific  gravities.  The  cal- 
culated distances  are  given  for  quartz  and  galena  grains  of  the 
same  diameter,  i.  e.,  d  =  4  m/m.,  and  for  a  galena  grain  of 
0.95  m/m.  diameter  which  has  the  same  settling  velocity  as  a 
quartz  grain  of  4  m/m.  diameter. 

It  may  be  seen  from  this  table  that  the  galena  grain  of  the 
same  diameter  as  the  quartz  grain  advances  rapidly  over  the 
quartz  grain  as  well  as  over  the  smaller  galena  grains  and  that 
the  galena  grain  of  equal  settling  diameter  with  the  quartz  grain 
advances  to  some  extent  in  the  beginning  over  the  larger  quartz 
particle  but  is  overhauled  after  two  seconds  by  the  larger  quartz 
grain. 

Table  3  shows  that:  (1)  The  smaller  of  two  grains  of  the  same 
specific  gravity  rises  higher  than  does  the  bigger  one,  and  (2)  a 
grain  of  lower  specific  gravity  of  the  same  size  will  be  raised 
higher  than  the  one  of  higher  specific  gravity,  if  the  ascending 
column  of  water  is  strong  enough  to  lift  the  grains.  (3)  A  grain 
of  higher  specific  gravity  of  two  grains  having  the  same  settling 
velocities,  rises  higher  than  one  of  lower  specific  gravity.  If  the 
ascending  column  of  water  is  not  able  to  lift  the  grains,  they  fall 
in  the  ascending  current  and  the  distances  become  negative  and 
even  then  the  grain  of  higher  specific  gravity  advances  more 
rapidly  than  the  grain  of  lower  specific  gravity,  just  as  in  a  quiet 
body  of  water. 

Table  4  shows  the  conditions  in  a  descending  current  of  water. 
With  low  velocities  the  galena  falls  faster  than  the  quartz  of  the 
same  diameter,  as  was  to  be  expected  from  observations  made  in 


THE  DEVELOPMENT  OF  COAL  WASHING 


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THE  DEVELOPMENT  OF  COAL  WASHING  23 

a  quiet  body  of  water.  With  higher  velocities  the  quartz  ad- 
vances at  first  and  only  after  0.15  seconds  does  the  'galena  take 
its  place  according  to  its  higher  specific  gravity. 

In  the  same  way  the  smaller  of  two  grains  having  the  same 
specific  gravity  falls  more  rapidly  in  both  currents  of  descending 
water  at  the  beginning,  but  after  respectively  0.05  and  0.25  sec- 
onds the  bigger  one  overhauls  the  smaller  and  quickly  passes  it. 

These  observations  are  important  for  a  clear  understanding  of 
the  process  of  jigging. 

Table  4  shows  furthermore  that  of  two  grains  having  equal 
settling  qualities  the  one  having  a  higher  specific  gravity  ad- 
vances more  rapidly  than  the  one  with  a  lower  specific  gravity 
and  that  the  difference  in  velocity  is  greater  in  a  current  having 
a  velocity  of  1  meter  than  in  a  current  of  only  0.25  meter  velocity 
per  second. 

From  the  results  of  his  calculations  Rittinger  developed  the 
following  theory  of  the  jigging  process: 

"On  the  down  stroke  of  the  plunger  the  ascending  column  of 
water  lifts  evenly  the  whole  bed  as  a  compact  body,  without  per- 
mitting the  smaller  and  lighter  particles  to  rise  appreciably 
higher  than  the  bigger  and  heavier  ones.  The  heavy,  thick  body 
of  the  material  resting  upon  the  screen  prevents  a  free  move- 
ment of  the  individual  grains.  Possibly  only  the  grains  of  the 
top  layer  follow  the  influence  of  the  ascending  water,  as  they 
have  freedom  of  action  upward. 

"On  the  downstroke  of  the  plunger,  the  descending  current 
forces  all  grains  to  move  downwards.  The  grains  occupying 
the  lower  strata  can  move  freely  and  follow  the  law  for  short 
timed  fall  under  moderate  velocities  of  the  descending  column 
(Table  4).  The  grains  having  a  greater  specific  gravity,  either 
large  or  small  ones,  advance  over  the  grains  having  lower  specific 
gravity,  and  collect  on  the  screen,  whereas  the  grains  with  lower 
specific  gravity  arrange  themselves  in  higher  strata.  The  grains 
in  the  upper  strata  follow  the  same  law  on  account  of  the  greater 
freedom  of  action  and  the  loose  condition  of  the  bed. ' ' 

Althans  admits  that  the  formulae  and  observations  of  falling 
bodies  during  a  longer  time  interval  and  the  average  velocities 
derived  therefrom  for  bodies  having  equal  settling  qualities  can 
not  be  used  for  the  study  of  the  jigging  process. 


24  COAL  WASHING 

In  all  jigs  an  intermittent  up  and  down  motion  in  quick  suc- 
cession with  only  small  lifts  and  drops  is  used.  To  accomplish 
an  effective  separation  the  lifting  of  the  material  must  be  carried 
on  under  a  sharp  impulse;  the  dropping  of  the  bed,  however, 
ought  to  be  as  far  as  possible  carried  on  under  free  settling  con- 
ditions. With  the  rapid  oscillations  between  an  upper  and 
lower  point  of  rest,  only  the  different  velocities  of  the  grains, 
acting  during  a  short  time  and  for  a  small  distance,  can  be  taken 
into  consideration. 

Althans  also  states  that  the  influence  of  the  lifting  impulse 
upon  the  bed  can  not  be  definitely  and  correctly  expressed. 

Hoppe  also  takes  the  stand  that  the  grains  of  higher  specific 
gravity  have  a  greater  tendency  to  sink  in  water  than  the  grains 
of  lower  specific  gravity,  so  that  in  any  case  they  advance  at  least 
at  the  beginning  of  the  down  stroke  of  the  plunger  over  the 
grains  of  lower  specific  gravity  and  are  thereby  separated  from 
them. 

This  explains,  without  considering  the  influence  of  the  im- 
pulse by  the  current  of  water,  the  general  action  of  a  jig.  Mun- 
roe,  whose  work  has  been  verified  by  Ladenburg,  obtained  the 
following  results  for  settling  particles  ' '  en  masse, ' '  i.  e.,  under 
hindred  settling  conditions: 

v  =  c  V  cKs-1)  meters  per  second. 

where  c  =  0.833  for  small  spherical  grains,  0.490  for  rounded 
grains,  0.536  for  angular  grains  of  uniform  size,  and  0.307  for 
large  spheres  moving  in  a  mass  of  small  spheres  when  the  differ- 
ence of  the  diameters  is  great. 

If  we  take  the  specific  gravity  of  pure  coal  at  1.35  and  that  of 
the  impurities  to  be  removed  at  1.50  we  get  the  following  ratio 
of  diameters  under  hindred  settling  conditions : 

Velocity  of  largest  particle  of  pure  coal : 


v  =  0.307  yd  (1.35-1) 
Velocity  of  smallest  particle  of  impurity: 


Vl  =  0.833  Vdj  (1.50-1) 

d        /0.833V     0.50 
Therefore  —•==[-       ~)x~    ~  =  10-5 
d,       V0.307/       0.35 


THE  DEVELOPMENT  OF  COAL  WASHING  25 

Therefore  it  would  appear  that  unless  the  specific  gravities  of 
the  ingredients  of  the  mixture  approach  one  another  quite 
closely,  it  is  not  necessary  to  size  the  material  in  a  detailed  man- 
ner. Indeed  Munroe  states  that  for  the  treatment  of  fine  stuff 
on  jigs,  close  sizing  is  a  positive  disadvantage,  as  the  concentra- 
tion of  the  fine  material  takes  place  in  the  small  interstitial  chan- 
nels forming  the  mineral  bed. 

Bring,  however,  has  shown  that  the  smaller  particles  of  the 
light  material  are  drawn  down  between  the  large  particles  by  the 
interstitial  currents  during  jigging. 

By  taking  the  above  ratio  of  diameters,  under  hindred  settling 
conditions,  we  see  that  if  we  wash  3  in.  screenings,  slate  as  fine 
as  H  in.  can  be  removed,  and  if  we  wash  1  in.  screenings,  all 
impurities  larger  than  10  mesh  will  go  into  the  refuse. 

The  above  assumed  specific  gravities  of  1.35  for  pure  coal  and 
1.50  for  refuse  were  selected  because  material  with  a  specific 
gravity  higher  than  1.50  contains  too  much  ash  to  permit  it  to  go 
in  the  washed  coal.  Should  such  material  contain  too  much  fixed 
carbon  to  be  rejected  with  the  refuse,  rewashing  must  be  applied 
in  order  to  get  a  clean  refuse  free  from  bone  coal. 

If  we  review  the  above  expressed  theories  of  jigging,  we  find 
that  two  main  opinions  exist  in  regard  to  the  action  in  a  jig. 

The  one  considers  that  the  action  in  a  jig  can  be  explained  by 
the  law  of  equal  settling  and  the  other  holds  that  this  law  is 
inapplicable,  but  that  only  the  rules  for  the  acceleration  of  solid 
bodies  falling  in  moving  water  can  be  considered  for  a  proper 
explanation  of  the  jigging  process.  Besides  this  we  find  differ- 
ences of  opinion  in  regard  to  the  forces  that  act  upon  the  bed 
and  in  regard  to  the  manner  in  which  these  forces  are  applied. 

It  is  not  very  probable  that  the  process  of  jigging  is  carried 
on  according  to  the  laws  of  equal  settling.  The  fact  alone,  that 
the  product  from  hydraulic  classifiers  and  ''V"  boxes  (Spitz- 
kasten)  can  be  treated  with  good  results  in  a  jig,  is  proof  that 
the  success  of  the  jigging  process  rests  upon  other  factors. 

It  is  not  easily  conceivable  that  a  mixture  of  grains  of  differ- 
ent specific  gravities  and  different  sizes,  which  has  been  pro- 
duced under  equal  settling  conditions  in  a  horizontally  flowing 
current  of  water,  either  with  or  without  the  use  of  an  upward- 
rising  column,  can  be  separated  under  the  same  physical  condi- 


26  COAL  WASHING 

tions  according  to  the  specific  gravities  of  the  individual  grains, 
by  using  only  a  different  kind  of  apparatus. 

The  scientific  investigations  of  Sparre,  Rittinger  and  espe- 
cially of  Professor  Richards  as  well  as  practical  experience,  have 
established  without  doubt  the  fact  that  in  the  law  of  equal  set- 
tling, not  only  the  specific  gravities  but  also  the  size  of  the 
grains  must  be  considered.  According  to  the  formula  for  equal 
settling:  dt  (s±-l)  =  d2  (s2-l),  the  diameters  of  the  grains  in  a 
mass  of  equal  settling  quality  are  in  reverse  proportion  to  their 
specific  gravities  minus  one. 

Practical  experience  teaches  that  in  a  body  of  grains  produced 
under  equal  settling  conditions  out  of  a  mixture  of  grains  of  dif- 
ferent specific  gravities  and  different  sizes,  smaller  grains  of 
higher  specific  gravity  are  found  beside"  larger  grains  of  lower 
specific  gravity. 

Only  when  grains  of  different  specific  gravities,  but  of  the 
same  diameter  are  subjected  to  the  process  of  equal  settling,  can 
a  separation  according  to  specific  gravity  be  accomplished. 
Then  the  formula  assumes  the  following  form :  dL  =  d2  and 
S1  =  s2.  The  separate  strata  contain  therefore  only  grains  of 
the  same  specific  gravity.  This  condition,  however,  can  never 
be  fulfilled  either  in  classifying  over  a  screen  or  in  a  hydraulic 
classifier. 

By  pressing  the  hand  flat  against  the  bed  of  a  properly  work- 
ing jig,  it  can  be  easily  felt  that  the  whole  bed  is  lifted  on  the 
down  stroke  of  the  plunger.  The  water  pushes  through  the  bed, 
the  separate  grains  whirl  one  against  the  other  and  assume  differ- 
ent positions.  On  the  up  stroke  of  the  plunger,  the  water  re- 
cedes and  the  sucking  action  can  be  easily  felt  on  the  finger  tips. 

It  is  doubtful  if  the  different  motions,  contingent  upon  each 
plunger  stroke  and  influenced  by  the  difference  in  the  shape  of 
the  grains,  can  ever  be  correctly  investigated  and  expressed  in 
a  mathematical  formula.  But  it  will  be  permissible  to  draw 
conclusions  from  the  behavior  of  regularly  formed  grains  in 
either  quiet  or  moving  bodies  of  water,  in  regard  to  the  possible 
actions  existing  in  a  jig.  Guided  by  the  general  rules  and 
maxims,  derived  from  the  behavior  of  regularly  formed  grains  in 
a  current  of  water  of  known  constant  direction  and  velocity,  it 
is  possible  to  form  a  correct  idea  of  the  jigging  process. 


THE  DEVELOPMENT  OF  COAL  WASHING  27 

Sparre  has  expressed  the  opinion  that  the  law  of  free  falling 
bodies  can  not  be  applied  to  the  process  of  jigging,  because  in 
this  process  only  small  distances  through  which  the  bodies  fall, 
can  be  considered  and  therefore  the  starting  velocities  and  the 
spaces  traversed  under  acceleration  must  be  taken  into  account. 
He  has  made  a  thorough  investigation  of  the  above  conditions 
and  has  arrived  at  the  following  conclusions : 

(1)  At  the  end  of  very  small  time  intervals  the  velocity  of 
grains  having  the  same  specific  gravity  is  the  same  regardless  of 
their  diameter. 

(2)  Of  two  grains  of  equal  settling  qualities  but  of  different 
specific  gravities,  the  one  having  a  higher  specific  gravity  ad- 
vances at  the  start  over  the  one  having  a  lower  specific  gravity. 
Borne  extended  these  investigations  to  include  the  behavior  of 
falling  bodies  in  an  ascending  column  of  water,  having  a  con- 
stant velocity.     He  demonstrated  the  following  principles: 

(3)  Of  two  bodies,  having  different  specific  gravities  and  such 
diameters  as  will  give  them  in  a  given  time  the  same  constant 
velocity,  the  one  with  a  lower  specific  gravity  has  at  the  start  a 
higher  velocity  than  the  one  with  a  higher  specific  gravity,  if  the 
bodies  are  moving  in  an  ascending  column  of  water  of  constant 
velocity. 

(4)  The  time  in  which  the  velocity  of  a  grain  will  become  uni- 
form decreases  in  direct  proportion  with  the  diameter  and  the 
specific  gravity  of  the  grain. 

Rittinger  also  investigated  the  jigging  process.  His  investi- 
gations confirm  the  correctness  of  Sparre 's  conclusions.  But  in 
regard  to  the  behavior  of  solid  bodies  in  an  ascending  column  of 
water  his  results  are  diametrically  opposed  to  those  obtained  by 
Borne.  While  the  latter  states  that  a  grain  of  lower  specific 
gravity  advances  faster  than  a  grain  of  higher  specific  gravity, 
Rittinger  is  of  the  opinion  that  a  grain  of  higher  specific  gravity 
has  at  the  start  a  higher  velocity  in  an  ascending  column  of 
water. 

The  behavior  of  solid  bodies  in  an  ascending  column  of  water 
require  therefore  still  further  investigation. 

Until  this  matter  has  been  fully  determined,  we  believe  that  it 
will  be  safe  to  agree,  as  Althans  and  Sparre  have  done,  with 
Borne  in  his  opinion  that  the  grains  of  lower  specific  gravities 


28  COAL  WASHING 

advance  faster  in  an  ascending  column  of  water  than  the  grains 
of  higher  specific  gravities. 

We  are  justified  doing  this,  by  consideration  of  the  fact  that 
a  grain  of  higher  specific  gravity  offers  to  the  impulse  of  the 
water  a  smaller  surface  than  a  grain  of  lower  specific  gravity 
having  equal  settling  velocities  and  that  it  will  therefore  assume 
the  velocity  of  the  ascending  column  of  water  much  slower  than, 
the  larger  grain  of  lower  specific  gravity,  which  offers  to  the 
current  of  water  considerably  larger  surface  to  act  upon. 

Professor  Richards  has  investigated  this  problem  quite  thor- 
oughly, using  an  apparatus  which  illustrated  graphically  the  be- 
havior of  the  different  grains.  In  all  the  diagrams  obtained,  the 
curve  for  the  specifically  lighter  materials  rises  higher  than  that 
for  the  denser  materials.  This  proves  the  correctness  of  Borne 's 
opinion. 

In  regard  to  the  behavior  of  solid  bodies  in  a  descending  cur- 
rent of  water  Rittinger  comes  to  the  following  conclusion : 

(5)  In  a  swift  current  the  specifically  lighter  one  of  two  grains 
having  the  same  diameter,  advances  at  the  start,  but  after  a 
short  time  interval  the  denser  one  overhauls  it. 

In  a  slow  current  the  opposite  is  the  case.  Of  two  grains 
having  equal  settling  qualities  the  denser  one  advances  over  the 
specifically  lighter  one. 

If  we  consider  the  behavior  of  the  materials  in  a  jig  in  accord- 
ance with  the  above  maxims  we  arrive  at  the  following  conclu- 
sions : 

On  the  downstroke  of  the  plunger,  the  water  passes  through 
the  screen  and  lifts  the  bed,  whereby  the  specifically  lighter  par- 
ticles will  rise  higher  than  the  denser  ones,  inasmuch  as  the  con- 
fined space,  the  direction  of  the  water  impulse,  the  shape  of  the 
grains  and  all  the  other  peculiarities  will  permit  a  movement  in 
this  direction.  At  the  next  moment  the  plunger  starts  on  its 
upstroke,  the  water  follows  and  the  grains  previously  lifted  by 
the  water  impulse,  fall  back  upon  the  screen.  While,  however, 
formerly  the  grains  with  a  lower  specific  gravity  advanced,  now 
the  opposite  takes  place.  The  denser  grains  drop  faster  than 
the  specifically  lighter  ones  and  therefore  the  distance  between 
grains  of  different  specific  gravities  becomes  greater  with  each 
plunger  stroke.  We  must,  however,  consider  that  the  time  given 


THE  DEVELOPMENT  OF  COAL  WASHING  29 

for  the  grains  to  sink  in  quiet  water  is  short.  The  water  is  only 
at  rest  at  the  change  of  stroke  direction.  This  period  of  rest  is 
shorter  or  longer  according  to  the  methods  used  in  operating 
the  plunger.  At  the  termination  of  the  change  of  stroke  and 
as  soon  as  the  plunger  obtains  its  full  velocity,  the  water  rushes 
through  the  bed  and  creates  a  suction  effect.  This  suction  is, 
according  to  maxim  5,  favorable  in  treating  materials  of  equal 
settling  qualities,  but  it  is  detrimental  for  screened  materials. 

The  scope  of  jig  work  is  broad.  Grains  from  10  mesh  to  3  in. 
in  diameter  can  be  treated  successfully,  if  the  difference  in  the 
specific  gravities  is  sufficiently  great  and  the  materials  adequately 
reduced  in  size  to  make  this  difference  in  specific  gravity  per- 
ceptible. 

On  account  of  the  above,  the  opinion  has  gained  ground  that  a 
close  classification  of  the  materials  to  be  jigged  is  not  necessary, 
but  Professor  Richards,  who  has  made  a  thorough  study  of  this 
problem  and  who  can  be  accepted  as  an  authority  on  this  sub- 
ject, says  in  his  book  on  "Ore  Dressing"  as  follows:  "The  gen- 
eral practice  of  the  day  seems  to  tend  towards  a  more  general 
application  of  the  English  system;  that  is  to  say,  towards  the 
use  of  the  jig  in  the  treatment  of  unsized  material  instead  of 
the  hydraulic  classifier.  While  the  treatment  of  material  sized 
between  wide  limits  is  possible  and  thoroughly  practicable,  still 
the  advantages  resulting  from  a  preliminary  sizing  cannot  be 
denied.  In  the  English  system  itself,  when  the  hutch  products 
of  one  jig  are  treated  upon  another,  we  are  making  use  of  a  pre- 
liminary sizing.  Again,  in  order  to  jig  an  unsized  product  suc- 
tion is  necessary  to  effect  a  separation,  and  suction,  as  has  been 
stated  previously,  results  in  cutting  down  the,  capacity  enor- 
mously. This  point  is  nowhere  better  exemplified  than  in  the 
case  of  the  pulsator  jig.  The  arguments  that  have  been  ad- 
vanced for  the  adoption  of  the  English  system  on  the  ground 
that  equal-settling  ratios,  many  times  larger  than  those  obtained 
under  free  settling  conditions,  exist  on  a  jig  bed,  have  been 
amply  disproved.  It  may  be  stated  that  both  systems  have  dis- 
tinct advantages  and  that  the  method  adopted  will  depend 
largely  upon  the  particular  conditions  existing  in  each  case." 

From  the  above  we  can  see  that  a  close  sizing  of  the  material 


30  COAL  WASHING 

to  be  jigged  is  not  absolutely  necessary  and  that  the  separation 
plant  can  be  much  simplified  and  arranged  on  economical  lines 
without  impairing  the  efficiency  of  the  jigging  process. 

In  coal  washeries,  Stewart,  Baum  and  Montgomery  are  wash- 
ing in  their  jigs  screened  coal  from  3  in.  diameter  down  with 
excellent  results.  If  we  consider  that  the  difference  in  the  spe- 
cific gravities  of  coal  and  slate  is  slight  as  compared  with  the 
difference  existing  in  metalliferous  ores,  we  must  come  to  the 
conclusion,  that  close  sizing,  except  in  a  few  special  cases,  is  not 
necessary  for  the  efficient  working  of  jigs  and  that  the  slight 
advantage,  which  might  be -gained  by  it,  does  not  warrant  the 
additional  initial  expense  and  increased  cost  of  operation  re- 
sultant upon  the  use  of  the  complicated  apparatus  required  for 
close  classification. 

The  methods  used  in  ore  dressing  can  not,  however,  be  em- 
ployed in  coal  washing  without  some  modifications.  A  coal 
washery  handles  a  large  tonnage  of  relatively  cheap  material. 
Especially  the  jig,  which  is  the  heart  of  every  washery,  has  to 
be  altered  considerably  in  order  to  handle  the  large  quantities  of 
a  material,  the  separate  particles  of  which  do  not  show  such  a 
marked  difference  in  specific  gravity  as  those  of  the  metalliferous 
ores.  A  great  number  of  quite  ingenious  pieces  of  apparatus 
were  developed  for  the  purpose  of  surpassing  the  simple  Hartz 
jig  in  both  quality  and  quantity  of  production. 

In  the  year  1879  there  was  such  an  abundance  of  jigs  that  a 
writer  of  that  period  exclaimed  rather  grievously:  "During 
the  last  50  years  so  many  partly  novel,  partly  improved  devices 
appeared  and  disappeared  that  the  metallurgists  became  abso- 
lutely confused." 

Since  that  time  the  makeup  of  the  jig  has  not  become  more 
complicated,  even  though  the  design  and  details  of  construction 
have  been  considerably  perfected.  It  is  not  possible,  within  the 
scope  of  this  book,  to  follow  all  the  ideas  advanced  and  which 
resulted  in  the  construction  of  some  wonderful  and  curiously 
shaped  pieces  of  apparatus.  It  is  only  possible  to  give  a  general 
outline  of  the  development  that  has  taken  place  and  to  observe 
which  ideas  have  justly  disappeared  and  which  deserve  our 
attention, 


CHAPTER  V 
THE  EVOLUTION  OF  THE  JIG 

The  first  jig,  used  in  the  year  1830  in  Saxonia,  was  a  pan  or 
basket  jig.  A  movable  sieve  full  of  coal  was  immersed  quickly 
in  a  tank  filled  with  water.  Fig.  3  shows  this,  the  first  jig  used 
for  coal  washing. 


Fig.  3.     Hand-operated  Jig  with  Moving  Screen 

By  depressing  the  lever  the  sieve  is  lowered  while  a  counter 
weight  lifts  the  sieve,  as  soon  as  the  pressure  on  the  lever  is  re- 
leased. This  jig  differs  from  our  modern  machines  in  the  fol- 
lowing characteristics:  (1)  Movable  sieve,  quiet  water;  (2) 
manual  operation;  (3)  intermittent  operation.  The  layers  of 
pure  coal  and  refuse  must  be  scraped  off  by  hand. 

To  modify  these  three  characteristics  in  a  suitable  manner  was 
the  first  step  in  the  evolution  of  the  jig.  In  order  to  clearly 
understand  the  manner  in  which  this  was  accomplished  we  must 

31 


32  COAL  WASHING 

keep  in  mind  the  relative  importance  of  these  characteristics  in 
regard  to  the  operation  of  a  jig. 

(1)  Movable  Sieve,  Quiet  Water.     The  sieve  is  an  unyielding 
body.     If  it  is  plunged  suddenly  into  a  body  of  water,  the  water 
is  forced  to  pass  uniformly  through  all  parts  of  the  sieve ;  i.  e., 
if  the  material  in  the  sieve  is  uniformly  distributed.     This  action 
of  the  water  loosens  up  uniformly  the  material  and  is  therefore 
favorable  for  the  separation  of  the  impurities.     On  the  other 
hand,  the  moving  of  the  sieve  full  of  coal  is  cumbersome.     It  re- 
quires considerable  power  and  the  wear  and  tear  is  quite  ap- 
preciable.    Therefore  the  process  of  separation  is  favorable  and 
the  mechanical  operation  unfavorable.     The  improvement  of  this 
operation  through  the  introduction  of  fixed  screens  and  pulsating 
water  currents  was  an  easy  step.     Attention  had  to  be  paid  that 
the  process  of  separation  should  not  suffer  thereby.     The  change 
in  the  moving  parts,  that  is,  from  sieve  to  water,  was  well  begun 
by  the  year  1840,  but  jigs  with  movable  screens  held  their  own 
until  the  seventies. 

This  can  be  accounted  for  by  the  existing  belief  that  the  so- 
called  "hydraulic  jigs"  could  not  separate  with  the  necessary 
uniformity.  Especially  in  England  jigs  with  moving  screens 
were  used  to  a  great  extent  and  in  America  Stewart  brought 
about  an  epoch-making  revolution  with  his  machine,  which  held 
the  field  for  a  long  time  without  a  competitor. 

(2)  Manual  Operation.     In  moving  the  sieve  by  hand,  it  was 
possible  to  loosen  up  the  coal  by  a  quick  stroke  and  to  give  the 
different   particles  plenty  time   to  stratify  during  a  slow  up- 
stroke.    This,  however,  depended  entirely  upon  the  skill  and  the 
conscientiousness  of  the  operator.     It  afforded  the  possibility  of 
good  separation  without  a  positive  assurance  thereof.     In  intro- 
ducing mechanical  operation,   a  guarantee   of  uniformity  was 
assured,  but  it  was  necessary  to  arrange  the  drive  in  such  a 
way  that  it  imitated  the  manual  operation.     This  point,  which 
is  no  longer  of  great  importance,  was  for  a  long  time  the  main 
issue  in  the  evolution  of  the  jig.     Much  interest  attached  to  this 
mechanical  problem,  and  some  curious  and  wonderful  devices 
were  developed. 

(3)  Intermittent  Operation.     The  only  constant  tendency  in 
the  evolution  of  the  jig  was  shown  in  the  effort  to  replace  the 


THE  DEVELOPMENT  OF  COAL  WASHING 


33 


intermittent  working  of  the  first  machines  with  an  apparatus 
permitting  continuous  operation.  It  did  not  take  long  to  see 
the  obvious  advantages  of  a  continuous  process,  giving  greater 
production,  independence  from  the  operator,  saving  of  labor  and 
a  reduction  in  the  cost  of  operation.  On  account  of  the  rela- 
tively low  value  of  coal,  compared  with  metalliferous  ores,  the 
coal  washeries  were  the  first  to  introduce  continuous  jigging  and 
even  if  they  borrowed  the  jig  from  the  ore  dressing  plants,  they 
deserve  full  credit  for  having  developed  successfully  the  con- 


Fig.  4.     First  Plunger  Type  Jig 

tinuous  machine.  Berard  must  be  considered  the  pioneer  of 
this  process.  He  introduced,  in  the  year  1848,  his  jig  in  coal 
washeries  and  established  thereby  the  basis  for  the  uninterrupted 
development  of  the  jigging  process  up  to  the  present  time.  Ten 
years  after  the  basket  type  of  jig  was  introduced,  a  machine  with 
fixed  screen,  the  so-called  "hydraulic  jig,"  made  its  appearance. 
Fig.  4  shows  this  device. 

The  removal  of  the  washed  coal  from  the  screen  and  the  clean- 
ing out  of  the  refuse  from  the  hutch  was  still  performed  by 
hand.  It  is  also  surprising  to  see  that  the  flow  of  water  from 
the  plunger  to  the  screen  compartment  was  throttled  or  choked 
down  by  making  an  extremely  narrow  opening  in  the  partition 
wall.  This  produced  a  swift  current,  which  was  furthermore 


34 


COAL  WASHING 


increased  by  the  faulty  ratio  of  the  plunger  area  to  the  screen 
area.  This  swift  current  of  the  wash  water  had  the  undesirable 
effect  of  loosening  up  the  materials  in  an  unequal  manner. 
Those  particles  nearest  to  the  plunger  compartment  were  more 
violently  agitated  than  the  materials  farther  away.  Instead  of 
reducing  or  entirely  suppressing  this  undesirably  swift  current 
of  water  by  making  the  opening  in  the  partition  wall  as  large 
as  possible,  inventors  galore  brought  forth  a  good  many,  nearly 
useless  and  often  complicated  devices  to  regulate  the  current  and 
action  of  the  water.  Fig.  5  shows  such  an  arrangement  in  a 
two  compartment  jig. 


Fig.  5.     Arrangement  of  Baffle  Plates  under  Jig  Screen 

Only  in  the  sixties  was  the  idea  brought  forward  of  transmit- 
ting the  pulsation  of  the  water  from  the  plunger  to  the  screen 
in  a  uniform  way,  by  making  the  area  of  both  these  compart- 
ments nearly  equal,  by  avoiding  all  contractions  in  the  opening 
between  them  and  by  rounding  off  the  bottom  of  the  hutch. 
Fig.  6  shows  some  of  the  proposed  forms  of  the  jig  tank,  from 
the  earliest  form  at  "a"  to  the  present  shape  at  "d  and  e." 

The  first  step  to  produce  uniform  operation  was  made  by  the 
French  engineer  Lacretelle  of  St.  Etienne,  as  shown  in  Fig.  7. 
He  used  a  grid  "b"  with  bars  spaced  about  4  in.  apart.  On 
top  of  this  is  fastened*  a  screen  "c"  with  square  openings  of 
0.5  m/m.  (about  a  50-mesh  screen).  This  screen  is  firmly  placed 


THE  DEVELOPMENT  OF  COAL  WASHING 


35 


between  two  others  of  about  8  mesh  for  protection.  At  a  dis- 
tance of  about  4  in.  above  the  screen  a  grating  "d"  is  located, 
having  the  bars  spaced  far  enough  apart  so  as  to  let  the  material 
pass  freely.  The  wooden  plunger  was  moved  by  hand.  The 


H  f-ft 


a  be  d  & 

Fig.  6.     Different  Types  of  Jig  Tanks 

position  of  the  grating  "d"  permitted  the  continuous  removal 
of  the  washed  product  and  the  operation  was  only  interrupted 
when  the  space  between  the  screen  "c"  and  the  grating  "d" 


Fig.  7.     Lacretelle  Jig 

became  filled  with  refuse.     This  was  shoveled  out  by  hand,  after 
removing  the  grating  "d." 

Even  if  this  apparatus  was  the  first  step  toward  continuous 
operation,  it  must  be  considered  as  being  quite  an  improvement. 


36 


COAL  WASHING 


Berard,  however,  only  one  year  later,  i.  e.,  in  1848,  introduced  a 
jig  with  continuous  removal  of  all  products.  He  based  the 
operation  of  his  jig  upon  two  other  ingeneous  ideas  to  wit: 
(1)  A  crank  and  link  motion  produced  a  rapid  down  stroke  and 
a  slow  up  stroke  of  the  plunger.  (2)  Fresh  water  was  intro- 
duced underneath  the  plunger.  This  tended  to  neutralize  the 
suction  effect  of  the  rising  plunger,  and  permitted  the  continual 
overflow  of  the  washed  coal  and  the  constant  renewal  of  the  wash 
water. 


Fig.   8.     Berard's  Differential  Motion 

The  differential  motion  used  by  Berard  is  shown  in  Fig.  8, 
while  Fig.  9  gives  the  construction  of  his  jig. 

The  screen  "a"  rests  in  the  jig  box  at  an  inclination  of  3  deg., 
sloping  from  front  to  back.  The  jig  box  is  connected  with  the 
plunger  compartment,  in  which  a  plunger  "d"  moves  up  and 
down.  On  the  up  stroke  of  the  piston  fresh  water  enters  the 
jig  box  at  "e"  which  partially  prevents  the  suction  effect  and 
reduces  the  settling  velocity  of  the  material  upon  the  screen. 
On  the  down  stroke  of  the  piston  the  valve  "f"  is  closed.  Dur- 


THE  DEVELOPMENT  OF  COAL  WASHING 


37 


ing  this  down  stroke  the  material  and  the  excess  water  are  auto- 
matically removed.  The  refuse  passes  out  through  the  slot  "i" 
and  over  the  dam  "h."  The  opening  of  the  slot  "i"  can  be 
regulated  by  a  slide  ' '  g. "  The  clean  coal  overflows  at  the  oppo- 
site side,  passing  over  a  dewatering  screen  "k." 

It  is  apparent  that  Berard's  jig  closely  resembles  in  its  fun- 
damentals our  most  modern  jigs. 

One  would  assume  that  Berard's  jig,  which  was  well  designed 
and  carefully  constructed  with  cast  iron  plunger,  cylinder  and 


Fig.  9.     Berard's  Jig 


steel  plate  jig  box,  would  have  influenced  the  whole  art  of  coal 
washing.  This,  however,  was  not  the  case.  Even  though  his 
jig  was  installed  at  various  washeries  in  France,  Belgium,  Ger- 
many and  England,  it  was  severely  criticised,  mainly  on  account 
of  the  idea,  that  the  wash  water  should  only  enter  the  jig  flow- 
ing in  one  direction,  so  that  the  separation  according  to  the 
specific  gravities  would  not  be  influenced  by  the  return  move- 
ment of  the  water. 

Meynier  in  the  year  1851  carried  through  this  idea  by  build- 
ing a  hydraulic  jig  in  which  the  reciprocating  plunger  was  re- 
placed by  a  double  acting  pump,  that  delivered  to  the  jig  box  a 
continuous  stream  of  water,  which  could  be  regulated  by  a  gate 


38 


COAL  WASHING 


valve  placed  between  the  pump  and  the  jig.  Meynier  reports 
on  the  operation  of  his  jig  as  follows :  ' '  The  working  of  the  jig 
is  so  perfect  that  no  particles  of  slate  can  be  found  in  the  washed 
coal,  neither  does  the  refuse  contain  any  coal. ' ' 

Nevertheless  he  changed  his  jig  later  on  as  shown  in  Fig.  10. 
He  divided  the  jig  in  two  compartments  and  following  the  second 
compartment  he  added  a  series  of  troughs  "  c  c  c "  for  the  pur- 
pose of  catching  some  of  the  middle  products. 


Fig.  10.     Meynier's  Washing  Machine 

Meynier 's  jig  must  have  had,  at  the  time  it  was  introduced, 
some  alluring  possibilities.  The  theoretically  perfect  separation, 
according  to  specific  gravities,  which  was  easily  accomplished  if 
we  accept  the  inventor's  assurance,  after  only  one  stroke  of  the 
piston,  and  the  fact  that  it  was  possible  on  account  of  its  quiet 
and  steady  operation  to  build  for  ten  times  the  capacity  of  other 
jigs,  made  a  good  many  friends  and  users.  This  jig  withstood 
all  competition  from  other  devices  for  the  same  purpose  up  to 
the  eighties. 

It  was  entirely  overlooked  that  the  machine,  even  if  several 
jigs  could  be  operated  by  one  pump,  was  cumbersome  and  used 
much  water  (25  to  40  gal.  per  stroke).  It  is  remarkable,  there- 
fore, to  find  the  following  opinion  in  Armangauds  Genie  Indus- 
triel  in  the  year  1852:  "The  apparatus  does  not  require  any 
repairs,  except  the  renewal  of  the  screen  plates." 

Meynier  made  the  mistake  of  giving  preference  to  minutely 
perfect  work  rather  than  simple  and  economical  operation. 
With  the  arrival  of  more  practical  experience  and  a  better  un- 
derstanding of  the  theory  of  coal  washing  his  jig  disappeared 
from  the  field. 

Meynier  was  not  the  only  one  to  use  a  current  of  water  flowing 


THE  DEVELOPMENT  OF  COAL  WASHING 


39 


in  one  direction  only  for  the  purpose  of  coal  washing.  But  his 
successors  neglected  even  the  necessary  requirement  of  a  continu- 
ous operation.  DeFrancy  and  Jarlot  thought  it  to  be  possible 
to  classify  and  separate  coal  ranging  from  dust  to  large  sized 
nut,  in  one  and  the  same  apparatus.  This  machine,  shown  in 
Fig.  11,  was  simple  indeed.  In  the  cylinder  "a"  a  perforated 
piston  "  b  "  can  travel  up  and  down.  On  starting  the  operation, 


d-  — 


Fig.  11.     DeFrancy  and  Jarlot's  Washing  Machine 

the  piston  is  raised  by  means  of  a  rack  and  pinion  "c"  to  the 
line  "d."  The  cylinder  is  now  filled  with  water  and  the  space 
above  the  piston  occupied  by  nut  coal.  By  disengaging  the  pin- 
ion through  the  lever  "  e  "  the  piston  is  caused  to  descend  slowly, 
thus  forcing  the  water  through  its  perforations.  The  coal  is 
thereby  held  in  suspension  above  the  piston.  On  the  end  of  the 
stroke,  when  the  piston  has  reached  the  bottom  of  the  cylinder, 
the  coal  is  stratified,  with  the  slate  on  the  bottom,  followed  in 


40  COAL  WASHING 

succession  by  the  coarser  coal,  the  fine  coal  and  the  sludge  on 
top.  By  throwing  the  pinion  into  mesh  again,  the  piston  ascends. 
The  water  above  it,  which  can  not  pass  through  the  dense  sludge, 
is  bypassed  through  the  valve  "f "  into  the  down  pipe  "g"  and 
thus  back  to  the  cylinder  "a."  The  piston  is  now  raised  to  the 
top  of  the  cylinder  and  the  layers  of  separated  material  being 
scraped  off,  in  succession  as  they  come  to  the  top  edge,  with  the 
scraper  "h."  Fine  sludge  which  passes  through  the  piston  is 
collected  at  "i"  and  can  be  discharged  into  the  space  "1" 
through  the  valve  "k."  This  apparatus — a  very  interesting 
toy — has  actually  been  used  in  many  washeries. 

Of  a  more  serious  character  was  the  washing  machine  of 
Lombard,  which  was  introduced  in  the  year  1856  in  the  Loire 
district.  Developed  from  a  jig,  it  took  only  its  outer  shape,  as 
its  operation  resembled  more  that  of  a  hydraulic  classifier. 
Other  types  of  classifiers  were  designed,  but  without  success. 
In  the  meantime  the  development  of  jigs  was  not  neglected  and 
besides  sensible  efforts,  a  good  many  foolish  ideas  were  intro- 
duced. Berard  without  doubt  had  the  right  basic  principle,  but 
in  the  fifties  a  good  many  different  ideas  prevailed.  Baure 
tried,  neglecting  continuous  operation,  to  obtain  large  capacities 
by  using  big  machines.  This  resulted  in  cumbersome  apparatus. 
He  used  a  screen  having  an  area  of  21  sq.  ft.  and  could  wash 
over  it  70  tons  of  coal  in  ten  hours,  against  only  seven  tons  with 
hand  operated  jigs. 

Gervais'  jig,  shown  in  Fig.  12,  had  a  steam  operated  pan  and 
a  steam  operated  scraper.  The  jig  basket  "z"  is  lowered  by 
means  of  the  steam  cylinder  "A"  and  covered  with  coal.  After 
a  certain  number  of  strokes  about  4  in.  in  length,  the  jig  basket 
is  lifted  out  of  the  tank  "L"  and  the  steam  operated  scraper 
"F"  pushes  the  different  layers  in  succession  onto  a  platform 
"M."  This  machine  is  only  described  here  to  show  that,  at  a 
time  in  which  the  Berard  jig  was  in  successful  operation,  it  was 
considered  feasible  to  build  a  jig  having  a  capacity  of  2%  tons 
per  hour,  employing  two  steam  cylinders. 

The  jigs  of  Marsais,  Robert,  Girard  and  Flanchon  as  well 
as  of  Ract-Madoux  were  operated  in  the  same  cumbersome  man- 
ner. They  are  only  mentioned  here  to  show  the  great  variety  in 
jig  construction  at  that  time.  In  the  year  1856  a  committee  ap- 


THE  DEVELOPMENT  OF  COAL  WASHING 


41 


pointed  by  the  Societe  de  I' Industrie  Minerale  investigated  and 
tested  all  of  the  above  named  jigs.  The  result  of  this  investiga- 
tion is  especialty  important,  because  it  proved  that  it  was  a  mis- 
take to  use  one  and  the  same  type  of  machine  for  all  sizes  of 
coal.  This  committee  recommended  the  use  of  the  Gervais, 
Marsais  and  Robert  jigs  for  nut  coal  only  and  considered  the 


Fig.  12.     Gervais's  Jig 

hand  jig,  the  steam  operated  jig  of  Baure  and  the  Berard  jig 
adapted  to  the  smaller  sizes. 

At  that  time  it  was  only  considered  necessary  to  build  the  jig 
intended  for  fine  coal  smaller  than  those  for  large  sizes,  to  in- 
crease the  number  of  strokes  and  to  decrease  the  height  of  water 
pulsation.  This  latter  was  arrived  at  by  making  the  piston  area 
of  fine  coal  jigs  much  smaller  than  the  screen  area.  The  difficul- 
ties caused  by  the  clogging  of  the  fine  screen  were,  however,  not 


42  COAL  WASHING 

overcome  and  the  washing  of  fine  coal  was  a  troublesome  affair 
until  in  the  beginning  of  the  seventies,  Luhrig  came  out  with  the 
idea  of  using  an  artificial  bed  on  the  screen.  This  type  of  jig, 
being  at  first  successfully  used  in  Silesia  and  Saxony,  quickly 
found  its  way  into  a  good  many  washeries  and  still  remains  the 
main  type  for  fine  coal  treatment. 

The  Luhrig  jig  was  first  introduced  into  America  by  Alex- 
ander Cunninghame,  who  built  an  experimental  Luhrig  washery 
at  the  City  Furnaces  of  the  Sloss  Iron  and  Steel  Co.  at  Birming- 
ham, Ala.  Later  on,  in  the  year  1894,  he  built  a  Luhrig  washery 
with  a  capacity  of  60  tons  of  coal  per  hour  at  Carterville,  111. 

The  development  of  the  jig  from  1850  to  1860  shows  that  the 
Berard  jig,  without  fundamental  changes  found  many  users,  but 
that  otherwise  a  great  variety  of  ideas  in  regard  to  methods  of 
operation,  capacity,  discharge  of  material,  etc.,  etc.,  caused  the 
construction  of  more  or  less  cumbersome  and  complicated  pieces 
of  equipment  and  that  with  the  apparatus  of  DeFrancey,  Jarlot 
and  Lombard  the  fundamental  principle  of  jigging  was  lost 
sight  of. 

In  the  following  decade  the  development  of  the  normal  jig 
advanced  steadily.  Sievers,  Rexroth,  Neuerburg  and  Revollier 
improved  the  methods  used  in  discharging  the  materials  and 
simplified  the  drive,  without,  however,  avoiding  some  mistakes  in 
details. 

A  description  of  these  machines  can  be  omitted  as  they  may  be 
considered  as  predecessors  of  the  modern  jig.  But  it  will  be 
worth  while  to  study  the  efforts  of  the  Frenchman  Evard,  who 
introduced  after  ten  years  of  hard  work  an  apparatus  which 
combined  all  the  experience  of  the  preceding  ten  years  and  was 
used  to  a  great  extent  in  France. 

The  Evard  washer  consists  of  a  classifier  and  a  circular  jig 
used  to  separate  the  middle  product  coming  from  the  classifier. 
The  classifier  starts  the  operation  and  avoids  the  necessity  of 
previous  sizing.  Fig.  13  shows  the  classifier. 

On  the  screen  piston  "a"  rests  the  mass  of  coal  to  a  height  of 
about  50  in.  Steam,  is  let  in  through  a  pipe  "b"  in  sharp  pulsa- 
tion, on  the  outer  circular  body  of  water  "c-c,"  in  the  begin- 
ning of  the  operation  with  slow  and  heavy  pulsations  which  be- 
come weaker  and  quicker  toward  the  end  of  the  process.  This 


THE  DEVELOPMENT  OF  COAL  WASHING 


43 


brings  about  the  following  stratification  of  the  material:  (1) 
Pure  slate;  (2)  a  middle  product  according  to  size;  (3)  clean 
nut  coal,  and  (4)  pure  sludge. 

Only  pure  slate  particles  pass  through  the  perforations  of  the 
piston.  This  fine  material  overflows  at  "D." 

After  the  jigging  has  been  completed  the  material  is  allowed 
to  rest  for  one  to  two  minutes,  after  which  the  screen  piston  is 


Fig.  13.     Evard  Classifier 

raised  by  the  hydraulic  cylinder  "e"  until  each  separate  strata 
enters  the  steel  ring  "f "  which  acts  as  a  scraper.  By  means  of 
the  hydraulic  cylinder  "g"  the  different  strata  are  pushed  off 
separately.  The  middle  products  are  carried  to  the  annular  jig 
shown  in  Figs.  14  and  15. 

The  annular  screen  "A"  rests  in  a  water' tank  "B"  on  rollers 
and  is  slowly  revolved  by  an  outside  gear.  This  annular  screen, 
however,  does  not  lie  in  a  horizontal  plane,  but  is  inclined  toward 
the  left  at  a  slope  of  1  in  30.  The  water  in  the  tank  is  carried  at 
such  a  level  that  the  lowest  point  of  the  screen  comes  about  8  in. 
below  and  the  highest  point  about  4  in.  above  the  water  level. 


44: 


COAL  WASHING 


The  space  inside  of  the  annular  screen  is  occupied  by  a  piston 
"C"  which  can  be  raised  quickly  by  a  cam  "E"  and  walking 


Fig.  14.     Annular  Jig  by  Evard.     Vertical  Section 

beam  "D."     The  speed  of  the  descending  piston  can  be  regu- 
lated by  a  dashpot  "F." 

Evard  believed  that  he  could  pull  some  of  the  smaller  slate 
particles  through  the  coal  if  he  reversed  the  usually  adopted 


Fig.  15.     Plan  View  of  Evard's  Annular  Jig 

method  of  slow  up  stroke  and  quick  down  stroke.     The  coal  is 
fed  to  the  jig  at  a  point  slightly  in  advance  of  where  the  screen 


THE  DEVELOPMENT  OF  COAL  WASHING  45 

enters  the  water  (at  about  "A"  in  Fig.  15),  and  is  uniformly 
distributed  over  the  screen  by  rakes  "G."  As  the  screen  dips 
deeper  and  deeper  into  the  water  the  coal  is  subjected  to  stronger 
and  stronger  pulsations  which  decrease  again  in  the  same  pro- 
portion after  the  screen  has  passed  the  deepest  point  of  immer- 
sion. In  travelling  forward  the  coal  rises  above  the  water  and  is 
partly  dewatered  by  the  suction  caused  by  the  pulsation.  Fur- 
ther on  the  stratified  materials  are  scraped  off  separately  by 
means  of  scrapers  "H-H"  set  at  different  heights.  The  middle 
products  are  further  treated  on  another  annular  jig.  This  jig 
was  built  with  a  diameter  of  32  ft.  6  in.  and  had  a  capacity  of 
60  tons  per  hour. 

The  Evard  machine  had  at  first  sight  a  good  many  alluring 
qualities.  Much  attention  was  paid  to  separating  the  materials 
according  to  the  specific  gravities  and  the  whole  process  of  wash- 
ing was  performed  in  only  two  or  three  machines.  The  cost  of 
operation  was  low  as  few  men  were  required  and  the  capacity 
was,  at  that  time,  considerable.  Each  separate  piece  of  appa- 
ratus, however,  had  a  good  many  complicated  mechanical  details. 
The  failure  of  a  single  one  of  which  caused  the  shut-down  of 
the  whole  washery.  These  difficulties  outweighed  the  superiority 
of  the  small  number  of  machines  and  the  urgent  call  for  sim- 
plicity sounded  the  death  knell  for  Evard 's  jig,  which  combined 
in  an  ingenious  manner  the  ideas  of  different  inventors. 

In  a  manner  similar  to  the  Evard  machine  all  other  types  of 
apparatus,  resembling  jigs,  disappeared  to  make  place  for  the 
normal  coarse  and  fine  coal  jig,  which  arrived  in  the  course  of  a 
steady  evolution.  This  has  finally  attained  its  present  position 
as  the  only  successful  apparatus  for  washing  coal. 

The  development  of  the  jig  was  carried  on  with  the  full  knowl- 
edge that  it  was  possible  theoretically  to  excel  the  work  of  the 
jig.  Practical  experience,  however,  has  taught  us  not  to  ex- 
change the  simplicity  and  assurance  of  uninterrupted  operation 
of  a  normal  jig  for  any  theoretically  more  perfect  but  practically 
defective  apparatus  having  an  excessively  minute  method  of 
operation. 


CHAPTER  VI 
OTHER  METHODS  OF  WASHING 

Before  the  further  treatment  of  the  products  from  the  jigs  can 
be  described,  it  will  be  necessary  to  illustrate  a  number  of  meth- 
ods of  separation,  developed  independently  from  these  machines. 
Some  of  these  methods  are  still  in  use  at  the  present  time  and 
others  have  been  abandoned.  Among  them  are  the  following : 

(1)  Trough  washers,  (2)  air  separators,  (3)  centrifugal  sepa- 
rators, (4)  separators  using  dense  liquids,  and  (5)  separators 
taking  advantage  of  the  different  shape  of  the  coal  and  slate 
particles. 

All  of  the  above  methods  will  be  treated  in  this  chapter  only 
when  and  in  so  far  as  they  were  used  as  an  independent  process 
in  the  treatment  of  the  total  mass  of  screened  coal.  The  use  of 
some  of  these  devices  for  other  purposes,  such  as  dust  collectors, 
dewaterers  and  sludge  separators  will  be  described  in  other  chap- 
ters of  this  book.  The  washing  of  coal  in  trough  washers  is 
equally  as  old  as  the  washing  in  jigs.  It  was  used  extensively 
in  France  and  Belgium  the  year  1840  onward. 

The  construction  of  a  trough  washer  is  ilustrated  in  Fig.  16. 


Fig.  16.     Trough  Washer 

The  raw  coal  is  fed  to  the  trough  by  a  launder  "a"  and  thor- 
oughly stirred  by  hand  in  the  first  compartment  "b."  In  this 
compartment  the  heavy  slate  and  the  largest  pieces  of  coal  settle 
to  the  bottom.  The  lighter  material  overflows  in  the  compart- 
ment "c,"  where  a  mixture  of  coal  and  lighter  refuse  settles  to 
the  bottom.  The  pure  coal  is  collected  in  the  compartment  "d" 
and  the  sludge  overflows  writh  the  wash-water  in  the  launder 
"e."  The  compartment  "b"  is  emptied  from  time  to  time  and 

46 


THE  DEVELOPMENT  OF  COAL  WASHING  47 

its  contents,  containing  about  80  per  cent,  of  good  coal  are  re- 
washed  in  jigs. 

The  main  characteristic  of  the  trough  washer  is  the  great 
amount  of  water  required.  This  often  reaches  a  quantity  three 
times  as  large  as  the  amount  of  coal  washed.  The  trough  washer 
did  not  produce  clean  refuse  and  required  jigs  for  rewashing 
the  material.  It  was  only  a  matter  of  convenience  that  kept 
these  washers  going  for  such  a  long  time.  The  lack  of  all  me- 
chanical devices  and  moving  parts  was  certainly  an  alluring 
characteristic  and  a  great  many  inventors  tried  to  improve  the 
trough  washer  in  such  a  way  that  they  could  compete  with  jigs. 
From  1850  to  the  end  of  the  last  century  the  trough  washer  of 


be  b 

Fig.  17.     Bell  Trough  Washer 


L 


"Bell"  was  extensively  in  use  in  England.  Fig.  17  shows  this 
apparatus. 

The  water  and  the  coal  flows  through  an  inclined  trough  "a" 
which  contains  riffles  "b."  These  riffles  catch  the  refuse.  The 
rakes  "c"  are  moved  to  and  fro  by  means  of  a  rod  "d."  The 
clean  coal  and  the  water  overflows  directly  into  a  railroad  car. 
When  sufficient  refuse  has  been  accumulated,  the  flow  of  water 
is  interrupted  and  the  riffles  are  raised  by  means  of  the  bell 
cranks  "e"  and  pull  rod  "f."  The  refuse  is  now  washed  down 
with  a  separate  stream  of  water  and  leaves  the  trough  through 
the  opened  gate  "g."  The  Bell  trough  washer  was  used  for 
fine  screenings  only,  as  at  that  time  the  nut  coal  was  only  sized 
and  not  washed  in  England. 

Elliot  introduced  as  late  as  1895  a  trough  washer  that  could 
be  operated  continuously.  This  is  shown  in  Fig.  18. 

This  washer  consists  of  a  steel  trough  60  ft.  long  having  a 
slope  of  1  in  15.  A  centrifugal  pump  "b"  delivers  the  water 
to  the  trough.  The  quantity  of  water  can  be  regulated  by  a 
valve  "c."  Any  excess  flows  back  into  the  pump  tank.  The 
coal  is  delivered  to  the  trough  by  a  launder  "d."  The  current 
of  water  is  regulated  in  such  a  way  that  the  clean  coal  flows  over 


48 


COAL  WASHING 


to  the  dewatering  Screen  "e,"  while  the  refuse  settles  to  the 
bottom  of  the  trough.  An  endless  scraper  conveyor  removes  the 
refuse.  The  height  of  the  flights  and  the  speed  of  the  conveyor 
are  dependent  upon  the  amount  of  refuse  and  the  size  of  the 


Fig.  18.     Elliot  Trough  Washer 


material.     The  water  passing  through  the  screen  flows  back  to 
the  pump  tank. 

In  the  year  1882  Bangert  built  a  simple  washing  machine  on 
the  principle  of  a  hydraulic  classifier,  as  shown  in  Fig.  19. 


Fig.  19.     Bangert  Washing  Machine 

The  crushed  coal  is  carried  in  a  stream  of  water  through  the 
box  "a."  The  refuse,  which  settles  to  the  bottom,  is  discharged 
through  a  movable  slot  "b,"  where  it  meets  a  current  of  water 
"e"  which  carries  back  into  the  box  "a"  any  good  coal  which 
may  have  passed  out  with  the  refuse.  After  passing  through 


THE  DEVELOPMENT  OF  COAL  WASHING 


49 


the  "V"  boxes  (spitzkasten)  "d"  the  coal  is  rewashed  in  jigs. 
Bangert  therefore  used  his  apparatus  only  as  a  preparatory  unit 
to  facilitate  the  work  of  the  jigs.  On  account  of  the  simple  con- 
struction and  the  efficiency  of  the  apparatus  it  is  used  even  at 
the  present  time  for  a  preparatory  treatment  of  excessively 
dirty  coal. 

Rhum  in  1885  introduced  in  Bohemia  a  belt  washer  illustrated 
in  Fig.  20. 


Fig.  20.    Rhum  Belt  Washing  Machine 

This  washer  operated  quite  successfully  with  coarse  coal.  The 
coal  is  fed  on  an  endless  belt  "c"  by  a  shaking  feeder  "b"  un- 
derneath the  hopper  "a."  The  wash  water  flows  over  the  belt 
"c"  out  of  a  box  "d"  and  can  be  regulated  by  a  gate  "e."  The 
water  current  carries  the  clean  coal  to  the  dewatering  screen 
"g"  and  the  refuse,  following  the  belt,  falls  in  the  tank  "  h,"  out 
of  which  it  is  removed  by  a  perforated  bucket  elevator.  It  is 
readily  conceivable  that  every  change  in  the  character  of  the 
coal,  in  the  velocity  of  the  water  current  and  the  speed  of  the 
belt  will  influence  the  result  of  this  machine.  It  is  not  possible 
to  compensate  these  different  qualities  as  readily  as  this  may 
be  done  in  a  jig,  where  the  frequent  and  repeated  pulsation  of 
the  water  take  care  of  any  irregularities  in  the  character  of  the 
coal. 

The  trough  washers  and  hydraulic  separators  have  been  treated 
in  some  detail,  because  efforts  to  use  them  as  an  independent 
washing  apparatus  have  continued  up  to  the  present  time.  It  is 


50  COAL  WASHING 

an  open  question  if  those  efforts  should  be  seriously  considered. 
There  is  no  doubt  that  the  operation  of  a  trough  washer,  if  freed 
from  moving  parts,  is  simpler  than  that  of  a  jig.  It  is  also  pos- 
sible to  reduce  the  immense  water  consumption,  by  circulating 
the  liquid  with  a  centrifugal  pump,  as  Elliot  did,  up  to  the  point 
when  the  .wash  water  becomes  too  thick.  This  occurs  mostly 
with  fine  and  friable  coal.  Practical  experience  has  taught  us 
that  all  trough  washers  introduced  up  to  the  present  time  suffer 
from  two  fundamental  disadvantages:  (1)  In  order  to  produce 
clean  coal  it  is  necessary  to  waste  a  good  deal  of  coal  with  the 
refuse.  This  disadvantage  can  be  remedied  by  rewashing  the  re- 
jected material,  but  this  destroys  the  simplicity  of  the  operation. 
(2)  The  effect  of  a  flowing  stream  of  water  upon  the  material  to 
be  separated  is  so  delicate,  while  the  least  change  in  the  velocity 
of  the  water  current  and  the  character  of  the  raw  coal  influences 
the  result  of  washing  to  such  an  extent  that  it  is  extremely  diffi- 
cult to  obtain  uniform  results. 

Were  it  possible  to  build  a  less  sensitive  apparatus  of  this  type, 
it  would  deserve  our  fullest  attention  and  recognition,  even  at 
the  present  time.  Thus  far,  however,  this  problem  has  not  been 
successfully  solved. 

At  present  trough  washers  are  restricted  to  use  as  a  prepara- 
tory apparatus  and  for  the  treatment  of  sludge. 

Air  Separators.  The  separation  of  coal  and  slate  by  means 
of  air  currents  can  be  treated  briefly. 

Air  is  too  sensitive  a  medium  to  be  used  in  a  commercial  way 
for  an  exact  separation  of  mixed  materials  containing  particles 
of  different  sizes.  The  absolute  weight  of  the  different  particles 
influences  the  separation  in  an  air  current  to  such  a  degree  that 
the  material  must  be  sized  closely  to  obtain  even  a  half  decent 
separation.  With  unsized  material,  the  air  does  not  produce 
clean  products,  but  only  those  having  equal  falling  velocities. 

These  can  only  be  separated  into  products  possessing  equal 
characteristics  by  a  subsequent  classification.  Furthermore,  the 
first  requisite  for  separation  by  air  is  absolute  dryness,  which  is 
not  one  of  the  characteristics  of  coal  as  it  comes  from  the  mine. 

In  the  year  1858  Schmitt  invented  an  ingenious  apparatus 
for  separating  ore  and  coal  by  means  of  air  currents.  He  did 
not  introduce  the  air  in  a  continuous  flow  but  intermittently. 


THE  DEVELOPMENT  OF  COAL  WASHING  51 

Gaetzschmann  was  of  the  opinion  that  if  an  air  current  was  used 
the  intermittent  puffs  of  air  could  have  no  advantage  and  that 
the  mechanical  devices  necessary  to  produce  them  would  com- 
plicate the  whole  apparatus.  It  also  would  be  difficult  to  obtain 
the  necessary  uniformity  in  the  separate  puffs  of  air. 

Schmitt's  apparatus  was  ingeniously  designed,  but  was  never 
introduced  into  actual  practice. 

An  extensive  air  separation  plant  was  built  in  the  year  1871 
at  the  mine  "  Rhein  Preussen."  Primarily  it  was  intended  to 
treat  the  coarse  coal  with  air,  but  this  proved  to  be  impossible 
and  only  fine  screenings  were  subjected  to  air  separation.  After 
a  good  many  changes  in  the  apparatus  there  remained  only  a 
device  which  blew  off  the  dust  from  the  coal.  For  this  purpose 
the  apparatus  proved  to  be  quite  efficient  and  was  used  until  the 
year  1890  at  different  mines  in  Westphalia.  The  results  ob- 
tained with  an  air  separator  showed  that  this  method,  as  an  inde- 
pendent process  of  separation,  was  not  successful  and  on  account 
of  the  character  of  the  medium  used  for  the  separation  it  holds 
out  no  promise  of  success  in  the  future. 

This  has  been  conclusively  proven  by  a  test  made  at  the  Serlo 
mine  near  Saarbriickeii.  This  test  was  carried  on  to  find  out, 
if  it  would  be  possible,  to  separate,  prior  to  washing,  the  fine 
particles  of  slate  and  fireclay  from  the  coal  by  means  of  com- 
pressed air  for  the  purpose  of  getting  a  clearer  wash  water. 

The  apparatus  used  for  this  test  was  arranged  in  such  a  way 
that  compressed  air  with  a  pressure  of  from  5  to  10  Ibs.  was 
blown  through  a  slot  24  in.  long  into  a  stream  of  fine  coal  (of  a 
size  that  had  passed  a  six  mesh  screen)  coming  from  a  hopper. 
The  separate  coal  and  slate  particles  were  blown  away  to  differ- 
ent distances  and  therefore  distributed  over  a  large  area.  The 
deposited  material  was  divided  into  zones  each  10  in.  in  width. 
Samples  were  taken  from  each  zone  and  separately  analyzed. 
The  results  from  this  test  showed  that  the  air  current  did  classify 
the  material  somewhat,  but  that  nowhere  was  there  a  distinct 
separation  of  coal  from  slate.  This  trial  therefore  gave  a  nega- 
tive result. 

Separation  by  Centrifugal  Force.  The  idea  of  using  centrifu- 
gal force  as  an  independent  means  for  separating  coal  from 
slate  prevailed  to  a  great  extent  during  the  period  from  1853 


52 


COAL  WASHING 


to  1863.  Froehiich  in  the  year  1853  built  a  barrel  apparatus. 
In  1858  Zemlinsky  designed  a  centrifugal  washer  in  which  he 
used  also  an  ascending  current  of  water.  Mackworth,  from 
1860  on,  used  his  centrifugal  washer  quite  extensively  in  England 
and  Belgium.  At  the  same  time  Imbert  constructed  an  appa- 
ratus having  a  rotary  motion,  and  in  the  year  1862  Cadiat  intro- 
duced his  centrifugal  washer. 


Fig.    21.     Cadiat    Centrifugal    Washer, 
Vertical  Section 


Fig.  22.     Cadiat  Centrifugal 
Washer.     Plan  View 


It  will  be  necessary  only  to  describe  this  latter  machine,  which 
as  the  result  of  ten  years  of  development,  demonstrates  with 
sufficient  clearness  that  centrifugal  force  for  the  purpose  of  com- 
mercial coal  washing  is  totally  inefficient.  This  force  requires 
an  apparatus  which  is  vastly  more  complicated  than  a  jig. 

Figs.  21  and  22  show  the  Cadiat  washer  in  section  and  ground 
plan. 

The  feed  pipe  "a"  with  butterfly  "b"  enters  the  washing 
tank  "c"  through  a  stuffing  box.  The  tank  revolves  around 
the  shaft  "d."  From  the  inner  space  "e"  the  coal  passes 
through  the  openings  "f"  in  the  outer  annular  space  and  is 
carried  to  the  circumference  by  the  disks  * '  g. "  The  lighter  par- 


THE  DEVELOPMENT  OF  COAL  WASHING  53 

tides  ascend  close  to  the  outer  wall  and  overflow  through  the 
openings  "h."  The  refuse  settles  at  the  bottom  against  the 
gates  "k,"  which  at  the  full  speed  of  the  apparatus  are  kept 
closed  by  the  centrifugal  force.  At  intervals  the  machine  is 
slowed  down.  Springs  attached  to  the  gates  then  overcome  the 
centrifugal  force  and  open  the  gates,  through  which  the  refuse 
is  discharged.  The  arms  "1,"  fastened  to  a  sleeve,  agitate  the 
material.  The  arms  are  revolved  by  means  of  gears  and  pinions. 
The  gear  meshing  with  the  pinion  "n"  is  held  to  the  pipe  "e" 
by  a  friction  ring  "o"  which  permits  a  certain  slippage  if  the 
resistance  of  the  coal  becomes  too  great. 

It  will  not  be  difficult  to  choose  between  a  jig  and  this  compli- 
cated machine,  especially  if  we  consider  the  rough  treatment  to 
which  the  coal  is  subjected  in  this  device. 

Centrifugal  separation  soon  disappeared.  At  present,  how- 
ever, centrifugal  force  is  used  for  drying  washed  coal,  in  the 
same  manner  as  it  has  been  employed  for  the  same  purpose  in 
other  industries,  especially  sugar  refining. 

A  device,  combining  the  actions  of  a  hydraulic  classifier  and  a 
centrifugal  machine  is  the  Robinson  washer,  which  since  the  year 
1890  has  been  used  extensively  in  England  and  is  still  in  opera- 
tion at  a  good  many  plants.  This  washer  was  introduced  into 
America  by  the  Jeffrey  Manufacturing  Co.  The  first  installation 
was  made  in  Alabama,  where  it  opened  the  field  for  coal  washing. 
Credit  is  due  to  the  untiring  efforts  and  the  mechanical  genius  of 
Erskine  Ramsay  for  the  rapid  introduction  of  this  washer.  In 
the  year  1912  there  were  still  eight  Robinson-Ramsay  washers, 
with  a  total  capacity  of  3,200  tons  of  coal  per  day  in  operation  in 
Alabama,  and  six  Robinson-Ramsay  washers  with  a  total  capacity 
of  4,360  per  day  in  use  in  Illinois,  besides  a  number  of  scattered 
installations  in  Georgia,  Tennessee  and  Ohio.  Fig.  23  shows  the 
Robinson-Ramsay  machine  as  installed  in  Alabama. 

In  the  washing  cone  "a,"  which  is  11  ft.  high  and  has  a  di- 
ameter of  11  ft.  6  in.  at  the  top  and  22  in.  at  the  bottom,  the 
stirring  arms  "b"  are  suspended.  These  arms  are  revolved  by  a 
bevel  gear  "c"  at  a  speed  of  from  20  to  24  r.p.m.  A  pulsometer 
"d"  takes  the  water  from  "e"  and  forces  it  through  the  pipe 
"f"  in  the  washing  tank.  The  unsized  raw  coal  is  fed  to  the 
washer  through  a  chute  "g."  The  separation  is  carried  on  by 


54 


COAL  WASHING 


the  influence  of  the  stirring  arms  and  the  ascending  current  of 
water  under  hindred  settling  conditions.  The  water  current 
carries  the  clean  coal  to  the  top  of  the  cone,  where  it  overflows 
on  the  screen  "h."  This  screen  is  4  ft.  6  in:  wide  by  15  ft. 
long  and  placed  at  an  inclination  of  30  deg.  It  is  made  of  man- 
ganese bronze  and  has  %  in.  round  perforations.  The  refuse 
sinks  to  the  bottom  of  the  cone  and  is  discharged  by  means  of 
two  steam-operated  slide  gates.  The  flight  conveyor  "i"  carries 
the  clean  coal  to  the  loading  chute  "k."  The  fine  coal,  which 


Fig.  23.     Robinson-Ramsay  Washer 

passes  through  the  screen,  falls  onto  a  shaking  screen,  where  it 
is  dewatered.  This  screen  has  perforations  of  #2  in.  in  diameter. 

In  some  installations  a  needle  slot  screen  is  used.  The  over- 
size from  this  screen  goes  to  the  loading  chute  and  the  drained- 
off  water  flows  in  the  Ramsay  sludge  tank,  which  is  shown  in 
detail  in  Fig.  24.  In  the  English  and  the  earlier  American 
plants  this  tank  was  merely  a  sump  for  the  -pulsometer.  But  the 
fine  material  carried  over  with  the  water  caused  rapid  destruction 
of  the  pipes,  valves  and  pulsometer.  It  also  accumulated  in  the 
tank  and  had  to  be  shoveled  out  every  day. 

Mr.  Erskine  Ramsay,  at  that  time  chief  engineer  of  the  Tennes- 
see Coal,  Iron  &  Railroad  Co.,  devised  a  tank  that  overcame  all 


THE  DEVELOPMENT  OF  COAL  WASHING 


55 


these  troubles.  The  Ramsay  sludge  tank  made  the  Robinson 
washer  an  efficient  and  successful  machine.  As  shown  in  Fig.  24, 
this  is  a  steel  tank,  cylindrical  in  section  at  the  top,  funnel  shaped 
at  the  bottom.  In  this  tank  is  a  circular  deflecting  plate  "a." 
The  water,  charged  with  fine  coal  and  impurities,  is  delivered 
into  the  top  at  the  center.  Thus  there  is  an  even  'distribution 
over  the  entire  surface  of  the  plate.  The  flow  of  the  water,  on 


Fig.  24.     Ramsay  Sludge  Tank 

entering  the  tank,  is  indicated  by  the  arrows.  The  fine  coal 
particles  are  carried  along  by  this  current  of  water  while  the 
impurities,  owing  to  their  greater  specific  gravity,  settle  out  of 
the  current,  as  indicated  in  the  illustration,  into  the  compara- 
tively still  water  below  the  level  of  the  mouth  of  the  pump-supply 
pipe  "  b, "  and  collect  in  the  bottom  of  the  tank.  From  here  the 
refuse  is  removed  by  means  of  a  valve  "c"  discharging  the 
sludge  into  a  trough,  by  which  it  is  carried  to  the  refuse  car. 
The  relation  between  the  diameters  of  the  deflecting  plate  and 


56  COAL  WASHING 

the  tank  is  a  detail  depending  on  the  amounts  of  coal  and  of  im- 
purities in  the  fines  and  on  the  difference  in  the  specific  gravity 
of  the  materials.  With  too  small  a  plate  the  impurities  will  go 
to  the  pump  with  the  coal.  With  too  large  a  diameter  the  coal 
will  not  be  carried  along  with  the  current  but  will  be  lost  with 
the  slate.  Once  regulated  for  a  given  coal,  the  results  are  very 
satisfactory. 

Fitted  to  this  tank  is  a  valve  for  supplying  the  fresh  water 
needed  by  the  washer,  automatically  regulated  by  a  float  "g." 
The  water,  freed  from  its  heavy  impurities  and  augmented  by 
the  necessary  make  up,  is  taken  by  the  pulsometer  through  the 
central  pipe  "b"  and  the  connections  "e,  e,"  and  pumped  di- 
rectly into  the  washing  cone.  This  is  an  innovation  on  the 
former  practice,  the  old  plan  being  to  pump  into  a  tank  40  to 
60  ft.  above  the  bottom  of  the  washer,  with  a  discharge  pipe  from 
this  tank  to  the  machine,  in  order  to  maintain  a  constant  head. 
In  the  newer  installation  the  same  object  is  accomplished  at  less 
expense. 

The  pipes  between  the  pulsometer  and  the  washer  are  con- 
nected to  a  standpipe,  80  ft.  in  height  and  open  at  the  top. 
This  acts  as  a  balance  on  the  inflowing  current  and  is  of  special 
advantage  when,  as  sometimes  happens  after  a  stoppage,  the 
material  in  the  washer  becomes  packed.  The  pulsometer  then 
forces  the  water  up  the  standpipe,  until  a  head  is  developed  suffi- 
cient to  force  a  way  through  the  obstructing  material.  The 
washer  is  driven  by  a  single  steam  engine  with  cylinder  10  x  16 
in.  One  man  does  all  the  work  at  this  machine.  He  must 
watch  the  engine  and  keep  it  as  well  as  the  other  machinery 
properly  oiled ;  operate  the  main  slate  valves  three  or  four  times 
an  hour,  and  also  the  sludge-tank  valve  and  load  the  washed  coal 
into  the  railroad  cars.  He  is  by  no  means  overworked  in  attend- 
ing to  these  duties,  and  will  have  ample  time  to  operate  the 
refuse  car.  For  the  same  capacity,  even  a  trough  washer  can 
hardly  excel,  if  it  can  equal,  this  labor  record. 

The  capacity  of  a  Robinson-Ramsay  washer  is  60  tons  of 
raw  coal  per  hour.  The  average  water  required  for  one  ton  of 
coal  is  35  gal.,  and  the  general  efficiency  of  the  machine  varies 
from  55  to  65  per  cent. 

The  advantages  of  the  Robinson-Ramsay  washer  are : — Low 


THE  DEVELOPMENT  OF  COAL  WASHING  57 

cost  of  installation,  low  cost  of  operation,  compactness,  ability  to 
treat  with  fair  results  unsized  coal.  The  disadvantages  are: — 
Low  efficiency,  small  capacity,  high  consumption  of  water  and 
an  unavoidable  loss  of  fine  coal. 

Separation  with  Dense  Liquids.  Berard  used  in  the  early 
days  of  coal  washing  solutions  of  chloride  of  zinc  and  calcium 
chloride  with  a  specific  gravity  of  1.4  to  determine  the  degree  of 
purity  of  the  washed  coal.  For  the  separation  of  coal  in  a  com- 
mercial way  Sir  Henry  Bessemer  proposed  in  the  year  1858  the 
use  of  dense  liquids.  He  advocated  solutions  of  ferro-chloride, 
barium-chloride,  potassium-chloride,  etc.,  etc.,  which  were  waste 
products  from  chemical  works.  Englinger  in  1859  proposed  the 
use  of  a  common  salt  solution  for  the  same  purpose. 

The  idea  itself  is  of  an  alluring  simplicity.  All  sizing  can  be 
avoided  and  no  mechanical  equipment  is  required.  It  is  only 
necessary  to  scrape  off  the  clean  coal  floating  on  the  surface  by 
means  of  rakes  and  to  dispose  of  the  refuse  by  means  of  a  screw 
conveyor  or  elevator.  It  would  also  be  possible  to  produce  three 
products  by  dividing  the  operation  into  two  stages.  In  the  first 
stage,  by  using  a  very  heavy  solution,  the  heavy  refuse  would  be 
separated,  and  in  the  second  stage,  by  using  a  lighter  solution, 
clean  coal  and  a  middle  product  could  be  obtained. 

Theoretically  the  above  is  correct,  but  practically  there  ap- 
peared difficulties  which  forced  an  abandonment  of  the  process. 
In  the  first  place  this  process  is  too  sensitive.  The  specific 
gravity  of  clean  coal  is  never  uniform  but  varies  between  1.2 
and  1.4.  The  refuse  shows  still  greater  variations,  i.  e.,  from 
1.5  to  3.5.  A  jig  does  good  work,  even  if  the  character  of  the 
raw  coal  changes  during  the  operation,  as  long  as  a  difference 
in  the  specific  gravities  of  the  material  exists.  The  process  of 
Sir  Henry  Bessemer  permitted,  however,  only  a  separation  of  a 
material  having  a  constant  and  uniform  character,  as  it  was 
based  upon  a  solution  with  a  fixed  specific  gravity.  Further- 
more, the  difficulty  arose  of  keeping  the  solution  always  at  the 
same  density.  It  was  also  necessary  to  free  the  washed  coal 
from  the  adhering  solution  by  a  subsequent  washing  or  rinsing 
with  fresh  water.  The  dense  liquids,  furthermore,  are  rather  ex- 
pensive, thus  increasing  the  cost  of  the  operation. 

For  coal  to  be  used  for  coke  making  this  process  is  entirely 


58  COAL  WASHING 

unsuitable,  as  the  chloride  salts  adhering  to  the  washed  coal  have 
a  most  injurious  effect  upon  the  brick  work  of  the  coke  ovens. 
The  process  has  been  tried  on  a  large  scale  in  Germany.  At  the 
Laura  and  Bolhorst  mine  near  Minden  a  large  and  expensive 
plant  was  built  for  the  purpose  of  using  this  process.  After  an 
exhaustive  test  under  actual  working  condition,  extending  over 
a  long  period,  it  was  abandoned,  as  the  process  proved  to  be  a 
total  failure. 

Separation  According  to  the  Shape  of  the  Particles.  The 
idea  of  separating  coal  from  slate  according  to  the  different 
shapes  of  the  respective  particles  is  based  upon  the  observation 
that  frequently  the  coal  breaks  in  the  shape  of  cubes,  whereas 
the  slate  occurs  in  flat  slivers.  This  observation,  however,  does 
not  hold  good  in  a  general  way.  The  slate  is  not  the  only  mate- 
rial to  be  removed.  Most  of  the  other  impurities,  such  as  pyrite, 
fire  clay,  and  bone  coal,  do  not  break  in  flat  pieces,  and  even 
slate  breaks  sometimes  in  big  lumps.  Under  certain  conditions 
coal  will  break  in  flat  pieces.  Any  method  of  separation  based 
upon  the  difference  in  the  shape  of  the  particles  can  not  be  used 
'  under  all  conditions,  but  only  as  an  expedient  at  mines  where 
the  different  shape  of  the  coal  and  slate  particles  is  reasonably 
constant  and  distinct. 

This  method  found  its  greatest  field  of  usefulness  in  the 
anthracite  region  of  Pennsylvania,  where  mechanical  slate  pick- 
ers can  be  found  in  many  breakers.  Separation  according  to  a 
difference  in  shape  of  the  material  has,  however,  a  promising 
future.  The  idea  can  not  be  rejected,  but  its  use  must  be  closely 
restricted,  to  avoid  failures,  such  as  have  occurred  during  the 
last  20  years. 

Separation  according  to  the  shape  of  the  pieces  is  only  possible 
under  certain  favorable  conditions,  i.  e.,  when  the  coal  and  the 
impurities  show  a  constant  and  sufficiently  great  difference  in 
the  form  of  their  respective  particles.  In  this  case  the  method 
has  great  advantages  as  it  does  not  require  water  as  a  separating 
medium.  In  all  other  cases  it  can  be  used  to  good  advantage  for 
special  purposes,  such  as  a  preparatory  cleaning  prior  to  wash- 
ing. The  Bradford  breaker  operates  upon  a  somewhat  similar 
idea. 


THE  DEVELOPMENT  OF  COAL  'WASHING          59 

Conclusion.  The  results  obtained  with  the  above  described 
methods  and  apparatus  can  be  summarized  as  follows: 

Trough  washers  are  only  adapted  for  the  very  fine  coal  or  for 
the  removal  of  fireclay  and  mud. 

Ascending  current  washers  deserve  the  credit  of  having  opened 
the  eyes  of  the  coal  operators  to  the  benefit  derived  from  improv- 
ing the  coal  by  washing,  but  on  account  of  low  efficiency  they 
have  been  supplanted  by  jigging,  which,  owing  to  its  simplicity 
and  satisfactory  efficiency,  is  far  ahead  of  all  other  methods. 

Air  separatprs  are  important  for  the  purpose  of  removing  and 
collecting  the^ust. 

Centrifugars'eparators  can  not  compete  with  the  other  meth- 
ods, but  centrifugal  force  is  used  to  some  extent  for  the  drying 
of  coal. 

Dense  liquid  separators  have  proved  failures. 

Separation  according  to  the  shape  of  particles  is  to  be  consid- 
ered only  in  special  cases.  It  can  be  used  as  a  preparatory 
process. 

The  products  resulting  from  the  foregoing  operations,  if  wet 
processes  only  are  considered,  are  washed  coal  from  3  in.  in 
diameter  down,  dirty  wash  water  containing  fire  clay  and  sludge 
and  refuse.  It  has  always  been  clear  that  the  first  two  of  these 
products  need  further  treatment  before  they  can  be  used  to  the 
fullest  advantage.  The  washed  coal  must  be  dewatered,  the 
wash  water  must  be  clarified,  and  the  sludge  removed.  This 
clarification  must  be  carried  on  to  such  an  extent  that  the  water 
can  be  either  returned  into  the  system  to  be  used  over  again  or 
that  it  can  be  let  run  away  without  polluting  streams  and  natural 
water  courses. 


CHAPTER  VII 
DEWATERING  AND  DRYING  OF  WASHED  COAL 

Washed  nut  coal  forms  a  loose  mass,  which  permits  an  easy 
and  quick  draining  off  of  the  water.  In  the  early  days  of  coal 
washing  the  coarse  material  was  dewatered  on  sqreens  placed  at 
the  overflow  from  the  jigs.  Later  on,  with  an  increased  capacity 
of  the  washeries,  draining  screens  in  front  of  each  jig  were  no 
longer  sufficient.  The  large  quantity  of  coal  to  be  handled  had 
to  be  stored  in  loading  bins  at  a  distance  from  the  jigs.  Devices 
for  dewatering  remained  in  principle  the  same.  Screens  with 
perforations  smaller  than  the  respective  sizes  of  coal  were  uni- 
versally employed  for  dewatering  of  the  washed  coal.  From 
the  simple  fixed  screen  progress  led  to  revolving  and  shaking 
screens.  If  the  coal  was  sized  after  washing,  one  portion  of  the 
sizing  screens,  which  are  mostly  located  on  top  of  the  loading 
bins,  is  used  for  the  dewatering  of  the  nut  coal  in  such  a  way 
that  the  water  is  drained  off  together  with  the  fine  screenings. 

Dewatering  of  Fine  Coal.  The  dewatering,  or  rather  drying, 
of  the  fine  coal  is  a  much  more  difficult  problem.  Fine  coal  if 
wet  forms  a  dense,  compact  mass  which  contains  too  much  water 
for  use  in  either  coke  ovens  or  boiler  furnaces.  The  wet  coal 
has  also  the  further  disadvantage  of  freezing  solid  in  the  winter 
time.  The  excess  water  increases  the  weight  of  the  coal  and  most 
consumers  object  to  paying  coal  prices  for  moisture.  The  drying 
of  the  fine  coal  has  for  a  long  time  been  a  sore  point,  and  even 
now  it  presents  great  difficulties.  For  this  reason  it  was  pro- 
posed to  screen  out  all  coal  below  ^  in.  and  mix  the  raw  fines 
with  the  washed  fuel. 

This  method  offers  the  simplest  solution  of  the  difficult  drying 
problem,  but  can  only  be  used  if  the  ash  and  sulphur  content  of 
the  washed  coal  are  thereby  not  materially  increased.  This, 
however,  is  hardly  to  be  expected,  as  no  material  improvement 
can  be  accomplished  by  washing  fines  below  10  mesh.  If,  how- 

60 


THE  DEVELOPMENT  OF  COAL  WASHING 


61 


ever,  the  addition  of  the  raw  fines  should  increase  the  ash  and 
sulphur  in  the  washed  coal  above  a  desired  amount,  it  would  be 
better  to  load  the  fines  in  the  raw  state,  or  to  use  them  for  fuel 
purposes. 

In  the  early  days  the  washed  fine  screenings  were  collected  in 
settling  tanks,  which  were  shoveled  out  by  hand  when  full.  The 
water  overflowed  the  tanks,  carrying  with  it  most  of  the  sludge. 
This  method  of  dewatering  was  inefficient.  In  addition  to  the 
high  labor  cost  it  was  necessary  to  subsequently  air  dry  the  coal. 
With  the  increased  capacity  of  the  washeries  there  was  not  suf- 
ficient time  available  to  do  this  and  it  became  necessary  to  find 


Fig.  25.     Hanrez  Centrifugal  Coal  Dryer 

better  methods.  Centrifugal  force  was  used  in  other  industries 
for  dewatering  and  it  was  quite  natural  to  try  it  for  the  same 
purpose  in  coal  washeries.  Hanrez,  in  1867,  invented  a  cen- 
trifugal coal  dryer  shown  in  Fig.  25. 

The  vertical  cylinder  "s"  having  a  perforated  mantle,  is  re- 
volved by  means  of  the  shaft  "a"  and  countershaft  "v."  This 
shaft  is  surrounded  by  a  sleeve  "b"  which  is  turned  by  means 
of  the  gears  "m  and  n"  in  the  same  direction  but  at  a  somewhat 
higher  speed.  On  this  sleeve  a  flat  helix  "d"  is  fastened,  which 
reaches  close  to  the  cylindrical  screen  "s."  The  screen  makes 
300  r.p.m.  and  the  helix  304  r.p.m.  The  coal  delivered  to  the 
machine  at  the  top  is  forced  against  the  mantle  of  the  cylinder 
'  V  and  at  the  same  time,  on  account  of  the  difference  in  speed, 


62 


COAL  WASHING 


carried  downwards,  where  it  falls  in  the  hopper  "r."  The  water 
passing  through  the  screen  runs  off  at  the  outside. 

No  evidence  can  be  found  in  contemporary  publications  that 
the  Hanrez  dryer  was  used  to  any  great  extent.  In  the  second 
part  of  this  book  the  development  of  centrifugal  dryers  will  be 
further  treated. 

In  the  seventies  the  fine  coal  was  sluiced  with  the  wash  water 
from  the  jigs  into  settling  tanks  and  recovered  by  elevators  hav- 
ing perforated  buckets.  The  washwater  carrying  with  it  the 
sludge,  overflowed  the  tanks.  This  method  had  the  decided  ad- 


A   A*  i\  A   A   1   A*  A 


Fig.  26.     Riehn,  Meinicke  and  Wolf  Washed  Coal  Dryer 

vantage  that  it  did  not  require  any  separate  apparatus.  It  was 
only  necessary  to  adapt  the  existing  elevators  to  this  purpose. 
Dewatering  elevators  were  greatly  improved  and  will  be  de- 
scribed more  in  detail  later  on.  At  times,  however,  the  in- 
ventors, never  idle,  were  carried  away  with  the  idea  as  demon- 
strated in  the  dryer  of  Riehn,  Meinicke  and  Wolf,  used  in  the 
year  1877,  in  the  Pilsen  district.  This  dryer  is  shown  in  Fig.  26. 
The  fine  coal  and  the  wash  water  are  sluiced  through  a  trough 
"a"  into  a  three-compartment  tank.  A  screw  conveyor  "b" 
carries  the  coal  forward  along  a  trough  the  bottom  of  which  is 
perforated.  The  water  in  the  tank  does  not  quite  reach  up  to 
the  screen  but  overflows  at  a  lower  level.  Behind  each  screen 
compartment  is  a  plunger  compartment,  similar  to  that  found 


THE  DEVELOPMENT  OF  COAL  WASHING  63 

in  a  jig.  The  plungers  have  a  quick  down  and  slow  up  stroke. 
On  the  down  stroke  the  air  is  forced  through  the  screen,  thereby 
cleaning  the  perforations  and  loosening  up  the  material  carried 
upon  it.  On  the  up  stroke  of  the  plungers  air  is  sucked  through 
the  coal,  carrying  with  it  the  water  adhering  to  the  particles. 
The  collected  sludge  is  discharged  at  the  bottom  of  the  tanks. 

This  apparatus  did  excellent  work,  but  on  account  of  the  ex- 
cessive cost  of  operation,  was  never  used  extensively.  During 
this  period  no  further  improvements  were  made  and  dewatering 
was  in  most  cases  accomplished  by  simply  letting  the  water  drain 
off  in  a  natural  way.  All  efforts  were  concentrated  to  make  the 
elevators  and  conveyors  as  far  as  possible  suitable  and  efficient 
for  the  purpose  of  dewatering  the  coal.  In  the  second  part  of 
this  book  the  modern  methods  of  coal  drying  will  be  described. 


CHAPTER  VIII 
WATER  CLARIFICATION 

The  methods  used  for  the  clarification  of  the  wash  water  and 
for  sludge  recovery  at  washeries  have  the  combined  purpose  of 
using  the  sludge  otherwise  carried  away  with  the  outflowing 
wash  water  and  of  obtaining  sufficiently  clear  water  to  be  used 
over  again.  The  wash  water  must  be  cleared  to  such  an  extent 
that  a  continuous  circulation  can  be  carried  on,  as  the  use  of 
fresh  water,  on  account  of  the  great  quantities  required,  would 
increase  the  cost  of  washing  to  a  considerable  extent.  Theoreti- 
cally the  process  of  clarification  should  be  carried  on  in  such  a 
way  that  only  sufficient  fresh  water  will  be  required  to  make  up 
for  the  loss  of  water  carried  away  with  the  washed  coal  and  the 
refuse,  and  also  that  lost  by  evaporation  and  leakage. 

The  methods  of  water  clarification  depend  entirely  upon  the 
nature  of  the  raw  coal.  Friable  coal  which  shatters  in  crushing 
and  which  contains  a  great  amount  of  fireclay  will  require  more 
extensive  and  carefully  designed  clarification  plants  than  a  raw 
coal  which  does  not  break  up  quite  so  fine  and  which  is  freer  of 
fireclay. 

The  oldest  and  simplest  means  of  clarifying  water  consisted  of 
large  settling  ponds  in  which  the  sludge  was  permitted  to  settle. 
After  one  pond  was  filled  up  with  sludge  a  new  one  was  installed 
or  the  walls  of  the  first  one  were  built  up  higher.  These  settling 
ponds,  however,  required  immense  ground  space  while  later  on 
two  clearing  basins  were  installed  which  were  used  intermit- 
tently. If  the  first  basin  became  filled,  the  water  was  run  into 
the  second  and  the  sludge  removed  from  the  first.  This  method 
of  water  treatment  is  shown  in  Fig.  27. 

The  dirty  water  flows  at  first  through  the  pipe  "a"  into  the 
tank  "b."  The  sludge  settles  to  the  bottom  and  the  clear  water 
overflows  at  "c."  When  the  tank  "b"  has  been  filled  with 
sludge  the  pipe  ' '  a "  is  turned  into  the  tank  "  d "  and  tank  "  b  " 

64 


THE  DEVELOPMENT  OF  COAL  WASHING 


65 


can  be  cleaned  through  the  opening  "e."  Similar  installations 
can  still  be  found  at  or  near  the  older  washeries.  The  main  dis- 
advantage was  that  the  sludge  had  to  be  shoveled  out  by  hand, 
which  is  a  costly  operation. 

The  use  of  mechanical  means  for  removing  the  sludge  was  the 
next  step  in  the  development  of  water  clarification.  In  the  year 
1880  " Spitzkasten"  were  used.  These  permitted  a  continuous 
operation.  The  sludge  was  withdrawn  from  the  apex  of  the 
Spitzkasten  and  carried  in  a  sluiceway  to  a  settling  tank  out  of 
which  the  sludge  was  conveyed  by  means  of  a  dewatering  elevator 


Fig.  27.     Settling  Basins  for  Water  Clarification 


to  the  fine-coal  bins.  The  outflow  of  the  sludge  from  the  Spitz- 
kasten was  regulated  by  a  valve  or  cock  and  was  hardly  ever 
continuous. 

In  some  installations  the  Spitzkasten  were  consolidated  into 
one  large  settling  cone  similar  to  the  Callow  tanks  used  in  ore 
dressing.  One  of  the  first  and  best  arrangements  of  this  kind 
is  the  Ramsay  sludge  recovery  tank  used  in  connection  with  the 
Robinson  washers.  To  make  the  process  of  sludge  recovery  still 
more  automatic  and  continuous,  narrow  and  long  settling  tanks 
were  installed  in  which  slow-moving  scraper  conveyors  or  even 
screw  conveyors  collected  the  sludge  which  had  settled  out  on 
the  bottom  of  the  tank.  On  the  end  opposite  to  the  inlet  of  the 
dirty  water  the  bottom  of  the  tank  was  depressed  to  form  a  sludge 


66 


COAL  WASHING 


pit  or  sump  out  of  which  an  elevator  carried  the  sludge  to  the 
fine-coal  bins. 

A  similar  device,  using  an  endless  belt  instead  of  a  scraper 
conveyor  has  been  invented  by  Bache  at  Kladno  and  is  shown 
in  Figs.  28  and  29. 


Fig.  28.     Bache  Settling  Tank  for  the  Recovery  of  Sludge.     Vertical 

Section 


Fig.  29.     Plan  View  of  Bache  Settling  Tank 

The  endless  belt  "c"  traveling  over  and  guided  by  the  rollers 
"fi-fz-fs^  and  g±"  in  the  direction  of  the  arrow  "p"  is  carried 
close  to  the  bottom  of  the  settling  tank.  The  dirty  water  enters 
the  tank  at  "a"  and  the  cleared  water  overflows  at  "b."  The 
sludge  settles  to  the  bottom  on  the  belt  and  is  carried  in  this  way 
out  of  the  tank.  A  revolving  helix  "d"  scrapes  the  sludge  off 
from  the  belt  and  drops  it  in  the  hopper  "e."  This  and  similar 
arrangements  were  partly  successful,  but  they  did  not  clear  the 
water  sufficiently,  so  that  subsequent  clearing  apparatus  was 
made  necessary. 

An  absolutely  quiet  body  of  water  is  required  to  settle  the  fine 
material.  It  is  a  fundamental  contradiction  to  employ  large 
tanks  in  order  to  neutralize  the  velocity  of  the  water  current  and 
to  destroy  at  the  same  time  the  still  water  body  by  moving  parts 
passing  through  it.  Efforts  were  made  to  disturb  the  water  as 
little  as  possible  by  reducing  the  speed  of  the  belts  or  conveyors. 
The  Bache  belt  had  a  speed  of  only  6  in.  per  minute,  but  this 


THE  DEVELOPMENT  OF  COAL  WASHING  67 

did  not  give  sufficient  capacity,  this  being  only  1,600  Ibs.  of 
sludge  per  hour.  In  America  flight  conveyors  are  largely  used 
to  collect  the  sludge  which  has  settled  in  the  settling  tanks. 

None  of  the  installations  described  in  the  foregoing  brought 
about  a  sufficiently  perfect  clarification  of  the  wash  water.  The 
moving  conveyors  disturbed  the  body  of  the  water  too  much  to 
permit  perfect  settling ;  the  water  overflowed  too  rapidly  to  drop 
all  of  the  fine  particles  and  the  resulting  sludge  was  too  liquid 
to  be  used  advantageously  without  further  dewatering.  The 
question  of  an  efficient  way  of  handling  and  preparing  the  sludge 
is  still  far  from  being  solved  in  a  satisfactory  manner. 

The  most  modern  type  of  sludge  recovery  and  water  clarifica- 
tion apparatus — one  that  promises  to  become  an  important  part 
of  any  coal  washery — has  been  borrowed  from  the  ore-dressing 
plants  where  it  has  been  used  satisfactorily  for  many  years. 

This  apparatus  is  the  Dor  thickener,  which  will  be  fully  de- 
scribed in  the  second  part  of  this  book. 

In  a  general  way  the  utilization  of  untreated  sludge  is  still  a 
sore  point  and  even  at  the  present  time  immense  -quantities  of 
fine  coal  are  simply  wasted.  This  problem  has  been  sadly  neg- 
lected and  110  other  part  of  a  coal  washery  can  show  such  primi- 
tive and  wasteful  arrangements  as  that  devoted  to  water  clarifi- 
cation and  sludge  recovery  notwithstanding  the  optimistic  state- 
ment of  G.  W.  Evans,  coal  mining  engineer  of  the  Northwest 
Experiment  Station,  Bureau  of  Mines,  at  Seattle,  who  said  at  the 
International  Mining  Convention  held  at  Vancouver  in  March, 
1919,  that  "A  coal  cleaning  plant  operating  along  most  modern 
lines  does  not  waste  very  much  except  the  color  ,in  the  water. 
Probably  some  enterprising  engineer  will  attempt  to  recover  the 
color  by  means  of  an  Oliver  filter." 

The  universally  customary  method  of  simply  wasting  the 
sludge  and  the  difficulties  and  expense  of  an  efficient  method  to 
recover  this  material  have  brought  about  the  fact  that  sludge 
recovery  has  not  been  developed  to  the  same  degree  as  the  sepa- 
ration of  the  coarser  coal.  The  latter  has  arrived  at  a  settled 
final  period,  whereas  sludge  recovery  requires  considerable  im- 
provements before  it  can  be  considered  as  satisfactorily  solved. 
With  the  majority  of  coal  washeries  the  sludge  recovery  is  only 
a  question  of  economics. 


CHAPTER  IX 
TREATMENT  OF  SLUDGE 

In  considering  the  development  of  sludge  treatment  we  can  not 
overlook  the  fact  that  in  the  early  days  no  sharp  and  fine  distinc- 
tion was  drawn  between  fine  coal  and  sludge.  At  the  present 
time  we  consider  as  sludge  only  such  material  as  is  carried  away 
with  the  water  overflowing  the  settling  tanks.  In  the  early  days, 
however,  the  term  sludge  had  a  broader  meaning  as  all  coal 
which  passed  through  a  6-mesh  screen  was  called  sludge. 

Fine  coal  and  sludge  were  not  separated  one  from  the  other 
after  leaving  the  jigs  and  the  dewatering  of  the  fine  material 
and  the  sludge  recovery  were  carried  on  in  one  and  the  same 
settling  tank.  This  classification  was  only  changed  after  it  was 
discovered  that  it  was  not  possible  to  clean  in  a  jig  the  fine  par- 
ticles held  in  suspension  in  a  current  of  water,  but  that  a  mate- 
rial from  Vs  to  %  in.  in  size  could  be  successfully  treated  in  a  jig. 
This  knowledge  brought  about  a  method  by  which  the  dirty 
sludge-carrying  water  was  clarified  separately  from  the  fine  coal, 
by  regulating  the  water  current  in  such  a  way  that  only  sludge 
was  carried  away  with  the  overflowing  water.  It  also  developed 
the  idea  that  if  sludge  should  be  treated  successfully  it  must  be 
through  the  employment  of  different  and  new  methods. 

Dor  was  the  first  to  build  an  apparatus  for  the  specific  pur- 
pose of  treating  sludge.  He  installed  his  apparatus  at  the 
Esperance  mine  near  Seraing,  Belgium. 

Fig.  30  shows  this  apparatus,  which  can  be  designated  as  a 
hydraulic  classifier.  The  results  obtained  with  this  apparatus 
are  of  interest  because  of  the  further  development  of  the  process 
of  treating  sludge. 

The  sludge  resting  on  the  screen  "a"  is  agitated  by  a  water 
spray  coming  through  a  sprinkler  head  "  b. "  The  sludge  pass- 
ing through  the  screen  drops  into  a  cylinder  "c,"  which  termi- 
nates in  a  conical  bottom.  A  fresh  water  current,  which  can  be 

68 


THE  DEVELOPMENT  OF  COAL  WASHING 


69 


regulated  by  a  valve  "d,"  ascends  in  this  cylinder  and  carries 
the  clean  coal  to  the  top  where  it  overflows.  The  impurities 
sink  and  are  discharged  at  "e." 

It  was  possible  at  the  Esperance  mine  to  reduce  the  ash  from 
42.5  per  cent,  to  from  12  to  15  per  cent.  At  another  mine  it 
was  noticed  that  the  overflowing  coal,  which  was  sluiced  over 
two  " Spitzkasten"  contained  in  the  first  Spitzkasten  12  to  11 
per  cent,  of  ash  and  20  per  cent,  of  ash  in  the  second  Spitz- 


Fig.  30.     Dor  Apparatus  for  the  Treatment  of  Sludge 

kasten.  Therefore  the  sludge  higher  in  ash  settled  in  the  second 
Spitzkasten. 

The  relatively  favorable  results  obtained  at  Esperance  can  be 
explained  by  the  fact  that  the  term  sludge  was  broadly  applied 
and  that  here  this  material  was  relatively  free  from  fireclay. 

The  results  obtained  in  the  Spitzkasten  showed  that  in  the 
treatment  of  fine  sludge  the  conditions  were  reversed.  Contrary 
to  the  action  in  the  separation  of  coarser  coal,  in  this  process  the 
coal  settles  first  while  the  impurities  are  carried  farther  forward. 
The  reason  for  this  phenomenon  can  be  easily  found.  When,  in 


70  COAL  WASHING 

the  preceding  separation,  all  fine  coal  which  was  not  carried 
away  as  fine  sludge  had  settled  out,  it  can  be  readily  seen  that 
the  same  must  also  hold  good,  but  in  a  higher  degree,  in  regard 
to  all  impurities  of  equal  or  smaller  size.  In  the  sludge  we  have 
therefore  only  the  finest,  almost  microscopic,  particles  of  impuri- 
ties, which  since  they  are  mostly  fireclay,  are  held  in  suspension 
in  the  water.  Coal,  however,  has  even  in  the  finest  sizes  a  dis- 
tinct sharp  granular  structure,  which  can  not  be  held  in  sus- 
pension in  water.  This  consideration  gave  the  idea  that  the 
treatment  of  sludge  must  consist  in  a  removal  of  the  fireclay 
and  not  in  the  separation  of  the  other  impurities. 

Artois,  in  the  year  1879,  designed  for  this  purpose  an  ex- 
tremely ingenious  but  complicated  apparatus  for  the  treatment 
of  sludge  and  fine  coal.  The  Artois  cradle  washer  was  further 
more  complicated  by  the  idea  of  limiting  the  use  of  fine  coal  jigs 
to  material  coarser  than  6  mesh  (4  m/m.).  The  inventor  there- 
fore was  compelled  to  arrange  his  apparatus  for  the  combined 
washing  of  fine  coal  smaller  than  6  mesh  and  also  sludge.  The 
apparatus  is  shown  in  Fig.  31. 

On  a  longitudinal  shaft  "a-b"  a  trough  is  suspended  in  a 
water  tank.  This  trough  has  two  compartments  "  A  "  and  ' '  B  " 
and  receives  a  double  motion.  By  the  lever  "c"  it  receives  an 
oscillating  crossways  movement,  while  the  cam  "d"  imparts  to 
it  a  sharp  lengthways  stroke  in  the  direction  from  "d"  to  "e" 
intermittent  with  a  slow  backward  motion  under  the  action  of  the 
spring  "e." 

The  material,  after  some  of  the  dirty  water  has  been  drawn  off 
in  a  concentrating  tank  is  delivered  onto  the  first  screen  "a." 
This  screen  has  a  fine  mesh.  The  sludge  is  carried  by  the  re- 
peated pulsation  of  the  apparatus  onto  the  plate  "f "  while  the 
dirty  water  passes  through  the  screen  and  is  discharged  from 
the  apparatus.  This  process  can  be  assisted  by  the  fresh  water 
sprinkler  "g." 

Passing  under  the  spreader  "z"  the  material  enters  the  sec- 
ond compartment  "B"  having  a  curved  bottom  plate  "h."  At 
"r"  is  a  screen  with  fine  mesh,  which  permits  the  continuous 
passage  of  fresh  water  from  the  water  tank.  This  fresh  water 
carries  the  clean  coal  over  the  steep  front  wall  "p"  into  a  com- 


TEE  DEVELOPMENT  OF  COAL  WASHING 


71 


72  COAL  WASHING 

partment  "i"  where  it  is  discharged  from  the  apparatus.  The 
impurities  settle  to  the  bottom  and  pass  through  the  perforated 
portion  of  the  plate  "  p  "  and  the  slot  "  q, "  the  opening  of  which 
can  be  regulated,  into  the  hopper  "1. "  The  first  compartment 
"A"  serves  only  as  a  sludge  washery,  whereas  the  compartment 
"B"  is  used  for  washing  of  the  fine  coal. 

Artois'  machine  found  extensive  use  in  Belgium,  France  and 
the  Sarre  district,  but  was  finally  abandoned.  For  the  washing 
of  fine  coal  the  simple  and  efficient  jig  was  sufficient  and  for 
the  treatment  of  sludge  alone  such  a  complicated  apparatus  was 
not  required. 

It  must  be  noted,  with  some  surprise,  that  the  idea  developed 
in  the  compartment  "  A "  of  Artois '  apparatus  was  not  immedi- 
ately followed  up.  It  was  not  until  1901  that  the  idea  was  taken 
up  again.  In  the  meantime  the  treatment  of  sludge  was  totally 
neglected. 

Harman,  in  the  year  1898,  had  the  idea  of  using  the  difference 
in  the  size  of  the  particles  of  coal  and  the  impurities  for  a  dry 
classification,  which  was  in  this  case  at  the  same  time  a  separa- 
tion. For  this  purpose  the  sludge  had  to  be  dried  and  this  was 
accomplished  by  means  of  a  screw  conveyor.  The  shaft  and  the 
flights  of  this  conveyor  were  hollow  and  heated  with  exhaust 
steam.  From  the  dryer  the  sludge  dropped  onto  a  screen  having 
about  80  meshes  per  lineal  inch.  This  divided  the  material  into 
two  products,  a  fine  floury  material  with  high  ash  and  a  coarser 
grained  material  with  less  ash. 

Nothing  has  been  heard  about  this  apparatus  in  actual  prac- 
tice. The  necessity  for  this  drying  of  the  sludge  and  the  diffi- 
culty of  screening  off  the  very  finest  material  made  the  apparatus 
useless  for  actual  operation. 

Karlik  in  1901  took  up  the  idea  of  Artois  in  his  belt  machine. 
Kohl  followed  him  the  next  year  with  a  screening  apparatus  and 
in  1905  Zorner  added  another  device  for  the  treatment  of  sludge. 
All  the  above  named  machines  use  the  principle  of  washing  off 
the  fireclay  with  the  dirty  water  and  as  they  belong  to  present- 
day  equipment,  they  will  be  described  more  fully  in  the  second 
part  of  this  book. 

It  must  be  noticed  that  these  devices  reduced  at  times  the  ash 


THE  DEVELOPMENT  OF  COAL  WASHING  73 

in  the  sludge  from  40  to  8  per  cent.  But  the  loss  of  coal  carried 
away  with  the  dirty  water  was  considerable  and  the  treatment 
of  sludge  is  a  problem  which  has  not  yet  been  solved  in  a  satis- 
factory manner. 


CHAPTER  X 
CONCLUSION 

The  sludge  disposal  is  the  final  process  in  coal  washing.  In 
regard  to  the  general  arrangement  of  a  coal  washery  it  must  be 
stated  as  important  that  it  has  always  been  considered  advan- 
tageous to  eliminate  as  far  as  possible  all  mechanical  means  for 
conveying  the  materials  from  one  unit  to  the  next.  The  different 
units  should  be  arranged  in  such  a  way  that  by  gravity  only,  or 
by  the  use  of  the  wash  water,  the  materials  can  be  moved  through 
the  whole  washing  plant. 

Where  washeries  are  located  on  level  ground  this  is  only 
partly  possible  and  elevators  and  conveyors  must  be  installed. 
In  the  early  washeries  the  mistake  was  made  of  spreading  them 
out  in  a  horizontal  direction  and  installing  between  each  unit 
a  separate  elevating  device.  This  necessitated  a  great  number 
of  individual  drives,  requiring  constant  repairs,  a  great  number 
of  spare  parts  and  consumed  excessive  power.  For  this  reason 
the  modern  washeries  are  built  with  the  units  arranged  verti- 
cally, thereby  concentrating  all  elevating  machinery  at  the  in- 
coming end  of  the  plant.  This  simplifies  the  machinery,  con- 
densing it  into  a  few  heavy  but  carefully  designed  elevators, 
driven  by  large  motors  that  can  be  operated  with  a  higher  effi- 
ciency than  a  great  number  of.  smaller  ones. 

The  modern  Anthracite  Coal  Breakers  are  built  on  this  prin- 
ciple and  the  raw  coal  is  elevated  from  the  mine  tipple  to  the 
top  of  the  breaker  by  means  of  a  gun-boat  running  on  an  in- 
clined track.  Only  lip-screen  material  and  waste  rock  are  re- 
elevated.  The  Alliance  breaker  of  the  Alliance  Coal  Mining 
Company  at  Kaska,  Pa.,  described  in  "Coal  Age,"  Number  14, 
Volume  18,  September  30,  1920,  is  the  most  modern  exemplifica- 
tion of  the  above  principle. 


74 


PART  II 

CHAPTER  XI 

PROCEEDINGS  AT  THE  MINE 

The  production  of  a  clean  product  makes  the  operation  of  a 
mine  more  economical  than  is  the  case  where  " everything  goes" 
and  is  also  more  favorable  for  the-  operation  of  a  coal  washery. 
The  mine  operation  becomes  more  economical  on  account  of  the 
smaller  amount  of  useless  waste  material  hoisted.  A  coal  wash- 
ery can  then  operate  under  more  favorable  conditions  because 
of  the  following  reasons:  (1)  The  capacity  of  each  individual 
piece  of  apparatus  becomes  greater  when  less  impurities  have  to 
be  removed.  (2)  The  percentage  of  washed  coal  or  the  yield 
from  a  given  amount  of  raw  material  is  increased.  (3)  Wear 
and  tear  of  the  apparatus  is  reduced  since  coal  does  not  cause  as 
much  wear  as  the  heavier  and  harder  impurities. 

Conditions  in  a  mine,  which  would  permit  the  hoisting  of  ab- 
solutely clean  coal  are  seldom  found.  Such  conditions  would 
reduce  the  process  of  preparation  to  a  simple  classification  into 
the  different  sizes.  Even  in  an  absolutely  clean  coal  bed,  this 
condition  will  not  be  completely  fulfilled.  The  possibilities  pre- 
sented by  the  natural  conditions  will  be  limited  by  the  goodwill 
of  the  miner  and  his  willingness  to  load  only  clean  coal. 

Consequently  the  aim  of  a  mine  superintendent  can  only  be 
to  prevent  as  far  as  possible  the  loading  of  dirty  coal.  The 
technical  means  of  doing  this  are  numerous  and  can  only  be  here 
indicated,  since  they  belong  to  the  art  of  mining  coal.  Some  of 
them  are:  (1)  Taking  advantage  of  the  slate  bands,  partings 
and  middlemen  in  undercutting  and  snubbing.  (2)  Mining  only 
the  clean  portion  of  the  bed  and  letting  the  mixed  parts  remain 
in  the  roof  or  on  the  bottom.  (3)  Careful  timbering  and  lag- 
ging if  necessary,  to  prevent  the  draw  slate  from  the  roof  being 
mixed  with  the  coal.  (4)  Careful  gobbing  to  keep  the  floors 
clean.  The  above  will  help  some,  but  it  will  never  be  possible  to 

75 


76  COAL  WASHING 

prevent  absolutely  the  mixing  in  of  some  impurities  with  the 
coal  at  the  face. 

The  loading  of  slate  with  the  coal  is  highly  convenient  for  the 
miner  and  often  of  pecuniary  advantage  to  him.  The  picking 
out  of  the  slate,  before  shoveling  the  coal  into  the  pit  car  takes 
time  and  it  is  more  convenient  for  the  miner  to  load  everything 
that  has  been  shot  down.  A  direct  advantage  of  loading  slate 
with  the  coal  exists  if  the  miners  are  paid  by  the  weight  of  coal 
they  send  out.  The  heavy  slate  secures  a  full  weight  even  if  the 
cars  are  not  topped. 

The  only  method  now  in  use  to  control  this  loading  of  slate  is 
the  collection  of  a  fine  for  loading  dirty  coal.  Such  a  docking 
system  furnishes  a  point  of  dispute  between  the  miner  and  the 
operator  and  a  just  and  effective  docking  rule  is  not  a  simple 
matter.  The  possibility  of  loading  clean  coal  is  never  the  same 
at  different  mines  and  even  in  the  same  mine  it  may  vary  at  the 
different  faces.  In  most  districts  a  docking  schedule  has  been 
prepared  in  which  the  fine  is  represented  by  a  deduction  of  a 
certain  amount  of  coal  while  in  some  cases  a  premium  is  given 
for  clean  coal  loaded.  The  docking  schedule  in  force  in  Alabama 
is  given  below : 1 

TYPICAL  DOCKING  SCHEDULE  IN  EFFECT  AT  PRATT  SEAM  COAL  MINES — 

ALABAMA 

1  can  slate  No  dockage 

1£  cans  slate  1     ton  dockage 

2  cans  slate  li  tons  dockage 
2£  cans  slate  2     tons  dockage 

3  cans  slate  3     tons  dockage 

4  cans  slate  4  tons  dockage 

5  cans  slate  See  Superintendent. 

A  can  of  slate  averages  45  Ib. 
A  mine  car  averages  1600  Ib. 

TYPICAL  DOCKING  SCHEDULE,  IN  EFFECT  AT  NICKLE  PLATE  SEAM  MINES, 

ALABAMA 

1  can   slate — give  miner  1  ton  coal 

2  cans  slate  No  dockage 
2J  cans  slate  1  ton  dockage 

3  cans  slate  2  tons  dockage 

4  cans  slate  3  tons  dockage 

5  cans  slate  See  Superintendent. 
A  can  of  slate  averages  45  Ib. 

A  mine  car  averages  1600  Ib. 

i  Supplied  by  courtesy  of  Erskine  Ramsay,  vice  president  Pratt  Consoli- 
dated Coal  Co.,  Birmingham,  Ala. 


PROCEEDINGS  AT  THE  MINE  11 

TYPICAL  DOCKING  SCHEDULE,  IN  EFFECT  AT  BIG  SEAM  MINES,  ALABAMA 

£  can   slate — give  miner  1     ton 

1  can   slate — give  miner  \  ton 

2  cans  slate — give  miner  £  ton 

3  cans  slate  No  dockage 

4  cans  slate  £  ton  dockage 

5  cans  slate  I  ton  dockage 

6  cans   slate  1  ton  dockage 

7  cans  slate  1£  tons  dockage 

8  cans  slate  See  Superintendent. 

A  can  of  slate  averages  45  Ib. 
A  mine  car  averages  3500  Ib. 

TABLE  5 


A  simple  example  will  show  the  importance  of  strictly  enforc- 
ing the  docking  rules. 

A  mine  hoists  daily  2,000  tons  of  dirty  coal.  The  limit  of 
slate  permitted  in  each  car  of  3,500  Ibs.  is  140  Ibs.  or  4  per  cent. 
This  means  (since  without  doubt  all  miners  will  take  advantage 
of  this  minimum)  an  involuntary  excess  hoisting  of  80  tons  per 
day.  When  now,,  on  account  of  a  change  in  the  conditions  or 
through  "kicks"  of  the  pit  committee  the  minimum  is  raised  to 
350  Ibs.  of  slate  per  pit  car  or  to  10  per  cent.,  the  excess  hoisting 
assumes  the  figure  of  120  tons  daily.  An  increase  of  6  per  cent, 
in  the  slate  allowance  carries  with  it  the  necessity  of  hoisting 
120  tons  of  rock. 

The  importance  of  the  above  in  its  effect  upon  a  coal  washery 
will  be  apparent  from  the  two  following  assumptions:  (a)  A 
washery  will  handle  per  hour  180  tons  of  raw  coal  (200  tons 
of  mine-run  will  be  handled  per  day  without  being  washed). 

1,800  X  96 

With  a  slate  allowance  of  4  per  cent,  we  will  get = 

100 

1,728  tons  of  washed  coal.  (It  is  assumed  for  the  sake  of  sim- 
plicity that  the  washed  coal  will  be  perfectly  clean.)  With  a 

1,800  X  90 

slate  allowance  of  10  per  cent,  we  will  get  =1,620 

100 

tons.  Thus  we  see  that  an  increase  in  the  slate  allowance  from 
4  to  10  per  cent,  will  reduce  the  output  of  washed  coal  by  108 
tons  per  day. 

(b)  A  washery  will  produce  per  day  1,800  tons  of  washed  coal. 


78  COAL  WASHING 

In  this  case  the  input  must  be  increased  by  the  amount  of  slate 
allowance.     With  4  per  cent,  of  slate  allowance  the  extra,  input 
1,800  X  4 

will  be: =  72  tons.     With  a  slate  allowance  of  10  per 

100 

1,800  X  10 

cent,  this  extra  input  will  amount  to  -         — 180  tons. 

100 

Thus  with  an  increase  of  6  per  cent,  in  the  slate  allowance  the 
input  is  increased  by  108  tons,  or  if  the  washer  can  only  handle 
180  tons  per  hour  the  working  time  of  the  washery  will  be  in- 

60  X  108 

creased  —          —  =  36  min. 
180 

The  above  examples  are  merely  schematic.  They  show  that  in 
case  (a)  an  immediate  loss  of  revenue  would  result  while  in 
case  (b)  the  cost  of  operation  would  be  increased  and  the  capac- 
ity of  the  washery  would  soon  be  insufficient  to  take  care  of  the 
excess  of  raw  coal. 

Of  no  less  importance  than  a  just  docking  rule  is  its  constant 
control. 

At  mines  where  picking  belts  are  used,  it  is  comparatively 
easy  to  pick  out  the  slate  from  each  pit  car  load,  if  the  picking 
tables,  which  in  this  case  become  docking  tables,  are  located  be- 
tween the  dump  and  the  screens,  so  as  to  catch  the  whole  pit 
car  load.  At  a  mine  in  Illinois,  the  coal  passing  over  the  dock- 
ing belt  can  either  go  to  the  crushers,  or  if  containing  too  much 
slate,  can  be  diverted  by  a  by-pass  into  a  railroad  car,  where  the 
slate  is  picked  out,  collected  and  tagged  with  the  check  number 
of  the  respective  miner  for  his  inspection.  The  speed  of  the 
docking  belt  and  the  intervals  between  dumps  is  so  regulated 
that  a  distinct  demarcation  line  between  two  successive  pit  car 
loads  is  maintained. 

At  mines  where  no  docking  or  picking  belts  are  installed,  a  few 
cars  are  picked  during  each  day's  run,  at  random.  These  are 
carefully  unloaded,  a  shovelful  at  a  time,  and  the  entire  contents 
of  each  car  closely  inspected.1 

i  "The  Ramsa^  Mine-Run  Sampler,"  by  H.  S.  Geismer,  Coal  Age,  Vol.  11, 
No.  14. 


PROCEEDINGS  AT  THE  MINE 


79 


The  weak  point  in  this  method  is  that  only  a  few  cars  can  be 
so  inspected  during  a  day's  operation,  and  many  miners  who  are 
willing  to  take  chances  load  cars  that  should  be  condemned, 
without  being  detected.  Another  point  to  remember  is  that  dur- 
ing the  morning  the  miners  generally  load  cleaner  coal  than  they 
do  late  in  the  evening,  when  they  are  cleaning  up  the  rooms. 
Thus  it  is  hard  to  judge  a  man's  average  work  by  a  single  car. 

Erskine  Ramsay,  first  vice  president  and  chief  engineer  of  the 


Fig.  32.     Ramsay  Mechanical  Sampler 

Pratt  Consolidated  Coal  Co.,  an  Alabama  corporation,  has  been 
experimenting  for  several  years  to  perfect  a  mechanical  sampler 
that  would  make  possible  the  testing  and  inspection  of  a  large 
number  of  mine  cars  without  the  necessity  of  actually  unloading 
their  contents.  The  experiments  were  carried  on  at  the  tipple  of 
the  company's  mine  at  Banner,  Alabama. 

The  contrivance  finally  perfected,  the  one  that  has  been  in- 
stalled as  part  of  the  regular  equipment  at  the  mine,  is  fully 
illustrated  in  Fig.  32.  On  July  18,  1916,  letters  patent  No. 
1,191,227  were  granted  to  Mr.  Ramsay  covering  the  invention, 


80  COAL  WASHINa 

The  sampler  consists  of  a  scoop  made  from  a  piece  of  16-in. 
wrought  pipe  about  13  ft.  long  from  which  a  piece  about  3  ft. 
long  in  the  top  half  at  one  end  has  been  cut.  This  scoop  will 
hold  approximately  a  100-lb.  sample.  It  is  placed  underneath 
the  regular  dump. 

This  scoop  is  pushed  out  by  means  of  an  air  cylinder  so  that 
it  comes  directly  underneath  the  stream  of  coal  as  it  is  being 
dumped  from  the  car.  By  means  of  the  twisted  guides,  the  scoop 
is  revolved  as  it  returns  from  its  position  under  the  chute  and 
empties  its  sample  of  coal  into  a  small  bin.  From  this  bin  the 
material  is  dropped  into  a  second  bin,  and  from  here  it  is  fed 
onto  a  small  shaking  screen. 

The  screen  separates  the  coal  into  two  sizes.  While  the  lump 
and  nut  coal  is  slowly  traveling  across  the  shaking  screen,  an  at- 
tendant picks  out  the  rock  and  other  foreign  matter.  This 
screening  of  the  sample  expedites  the  work  of  the  picker  and 
makes  it  more  effective.  No  attempt  has  been  made  at  Banner 
to  determine  the  amount  of  rock  in  the  fine  coal,  but  if  it  should 
appear  that  some  of  the  miners  were  shooting  up  their  slate  so 
as  to  be  able  to  load  it  out  without  being  detected,  the  screenings 
from  the  sample  could  easily  be  tested  by  the  float-and-sink 
method.  After  finishing  with  each  sample,  the  attendant  weighs 
the  rock,  slack  and  lump  coal  and  records  the  percentage  of  each. 

The  object  in  having  two  bins,  one  above  the  other,  is  to  pro- 
vide a  storage  for  one  sample  while  a  second  is  being  fed  out 
slowly  onto  the  shaking  screen. 

One  man  operates  the  entire  mechanism,  obtaining  as  many  as 
100  samples  in  a  shift  besides  picking  out  the  slate,  weighing  the 
lump,  slack  and  slate  and  making  the  necessary  record.  This 
record  when  carefully  kept  enables  the  operator  to  keep  a  close 
tab  on  his  miners  and  also  allows  the  miner  to  compare  notes 
with  his  fellows,  which  makes  it  possible  for  the  conscientious 
miner  to  get  credit  for  his  work. 

When  the  scheme  was  first  proposed,  some  were  of  the  opinion 
that  the  100-lb.  sample  might  not  be  representative  of  the  entire 
amount  of  coal  in  the  tram-car;  but  the  results  obtained  from 
the  device,  when  checked  with  rejections  from  the  washer,  seem 
to  indicate  that  the  sampling  is  fairly  representative.  It  is  well 


PROCEEDINGS  AT  THE  MINE  81 

to  recall  that  a  100-lb.  sample  from  a  mine  car  is  a  relatively 
large  amount,  compared,  for  example,  to  the  average  sample 
taken  from  a  railroad  car  under  the  methods  at  present  in  vogue. 

The  accompanying  charts  in  Table  6  show  graphically  how  the 
percentage  of  rock  in  the  coal  was  reduced  by  the  installation  of 
the  sampler.  The  device  was  installed  on  March  20,  but  the  men 
were  not  notified  until  the  twenty-sixth  day  of  March. 

Chart  A  gives  three  curves,  all  of  them  estimated  as  percent- 
ages of  the  entire  output  of  the  mine  covering  the  period  from 
March  19  to  June  1. 

Curve  No.  3  shows  the  total  amount  of  refuse  taken  from  the 
coal  after  it  is  loaded  by  the  miner  expressed  as  a  percentage  of 
the  entire  output;  it  was  obtained  by  adding  the  weight  of  the 
refuse  separated  at  the  washer  (curve  No.  2)  and  the  weight  of 
the  refuse  thrown  out  by  hand  from  the  picking  tables  (curve 
No.  1). 

All  the  coal  as  it  comes  from  the  mine  is  screened;  the  lump 
and  nut  is  passed  over  picking  belts,  and  the  slack  is  taken  to 
a  coal  washery. 

The  curve  on  Chart  B  is  based  only  on  the  mine  cars  that  were 
actually  sampled  during  the  same  period. 

Charts  C  and  D  give  the  results  as  affecting  two  individual 
check  miners,  selected  at  random  from  the  tonnage  sheets. 

Not  only  was  the  percentage  of  total  refuse  cut  down  from  20 
to  8  per  cent.,  but  the  number  of  slate  pickers  on  the  picking 
belts  was  reduced  by  half. 

The  particular  sampling  device  as  installed  at  Banner  (shaft 
equipped  with  the  Ramsay  revolving  dump),  could  not  be  in- 
stalled in  connection  with  all  tipples.  Consequently,  Mr.  Ram- 
say has  worked  out  a  device,  for  which  he  has  applied  for  a  pat- 
ent, that  will  be  adaptable  to  tipples  equipped  with  horn  or 
crossover  dumps. 

The  method  of  handling  the  sample  after  it  has  been  procured 
is  practically  the  same  as  at  Banner,  but  the  method  of  obtaining 
it  is  entirely  different.  A  series  of  small  openings  in  the  chute 
bottom  is  covered  with  movable  plates  controlled  by  levers  con- 
veniently arranged  so  that  one  man  can  operate  them.  This  ar- 
rangement makes  it  possible  to  take  several  samples  from  one 


82 


COAL  WASHING 


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PROCEEDINGS  AT  THE  MINE 


83 


car  or  several  samples  from  a  trip  of  cars  dumped  in  rapid  suc- 
cession. Figs.  33,  34,  35  and  36  show  different  views  of  this 
apparatus. 

This  machine,  with  no  attention  from,  or  delay  to,  the  dumper, 
and  therefore  without  reducing  the  usual  tipple  dumping  capac- 
ity, automatically  takes  individual  samples  of  100  to  200  Ibs. 
run-of-mine  coal  from  each  mine  car  sampled,  the  samples  being 


Fig.  33  Fig.  34 

Ramsay  Mechanical  Sampler  for  Cross-over  dumps 

taken  from  the  stream  of  coal  as,  and  when,  it  flows  from  the 
mine  cars.  A  record  of  the  miner's  check  taken  from  each  mine 
car  sampled  is  kept.  This  record  shows  the  percentages  of  rock, 
coal  and  slack  found  in  each  sample.  The  sample  taken  from 
the  mine  car  is  assumed  to  be  of  the  same  quality  as  that  of  the 
entire  car,  just  as  is  the  case  with  sampling  railroad  cars.  In 
this  way  the  mine  management  knows  every  day  exactly  the 
quality  of  coal  each  and  every  individual  miner  is  sending  out. 
Three  men,  one  operator  and  two  pickers,  do  everything  con- 
nected with  taking  the  samples,  from  a  large  proportion  of  the 
total  mine  cars  dumped  daily  in  addition  to  picking,  weighing 


84 


COAL  WASHING 


and  recording  the  results.  The  operator,  stationed  on  the  tipple 
floor,  sees  the  dumping  of  the  mine  cars  and  at  the  same  time 
observes  and  controls  the  operation  of  the  machine  taking  the 


Fig.  35  Fig.  36 

Ramsay  Mechanical  Sampler  for  Cross-over  Dumps 

samples,  weighing  and  recording  the  component  parts  of  the 
treated  sample.  The  pickers  do  nothing  but  pick  the  samples. 
This  machine  has  a  capacity  of  taking  and  treating  samples  at 
the  rate  of  one  per  minute. 


CHAPTER  XII 

INTERMEDIATE  UNITS  BETWEEN  THE  SCREENING 
PLANT  AND  THE  WASHERY 

A.     RAW  COAL  STORAGE  BIN 

A  raw-coal  storage  bin  by  all  means  should  be  included  in 
every  coal  washery,  and  this  should  be  made  as  large  as  possible 
for  the  following  reasons : 

1.  Every  interruption  in  hoisting  coal  will  stop  the  washery 
if  a  certain  reserve  cannot  be  supplied  during  such  periods. 

2.  The  foregoing  holds  true  for  an  interruption  in  the  opera- 
tion of  the  screening  plant. 

3.  If  the  operation  of  the  washery  is  interrupted,  the  presence 
of  a  raw-coal  storage  bin  will  permit  the  screening  plant  to  be 
run  until  the  raw-coal  bin  has  been  filled.     Otherwise  the  screen- 
ing plant  must  be  shut  down  and  in  connection  therewith  the 
hoisting  of  coal  must  cease  and  the  working  of  the  whole  mine 
must  be  stopped. 

4.  A  coal  mine  does  not  deliver  the  coal  regularly  during  the 
whole  day.     In  the  morning  the   hoisting  is  rather  slow   and 
speeds  up  until  the  middle  of  the  day.     It  is  apt  to  slow  down 
toward  the  evening.     For  the  proper  operation  of  a  washery  a 
regular  supply  of  coal  is  of  the  greatest  importance.     A  good 
sized  raw-coal  bin  permits  drawing  off  of  some  coal  left  over  from 
the  preceding  day  during  the  period  of  slow  hoisting  and  the 
filling  up  of  the  bin  when  more  coal  is  hoisted  than  the  washery 
can  handle. 

5.  If  on  account  of  heavier  hoisting  than  figured  upon  or  on 
account   of  insufficient   capacity  of  the  washery   more   coal   is 
mined  than  the  washery  can  take,  a  large  raw-coal  bin  will  make 
it  possible  for  the  washery  to  consume  the  daily  output  of  the 
mine  by  working  overtime. 

In  consideration  of  the  reasons  just  enumerated,  the  raw-coal 
storage  bin  demands  the  fullest  consideration  as  it  is  an  impor- 
tant equalizer  between  the  mine,  the  screening  plant  and  the 

85 


86  COAL  WASHING 

washery.  If  it  is  not  possible  to  build  a  sufficiently  large  bin 
between  the  screening  plant  and  the  washery,  it  will  become 
necessary  to  build  one  or  more  reserve  bins  at  some  convenient 
place.  Conveyors  must  be  installed  to  carry  the  coal  between 
these  bins  and  from  them  to  the  washery.  The  different  units 
between  the  screening  plant  and  washery  are  the  conveyors  from 
the  screening  plant  to  the  storage  bin,  the  storage  bin  proper 
and  the  conveying  system  from  the  bin  to  the  washery. 

In  the  above  it  has  been  assumed  that  the  mine  and  the  wash- 
ery are  located  in  close  proximity.  If,  however,  the  washery, 
as  is  often  the  case,  is  located  at  some  distance  from  the  mine  or 
if  a  central  washing  plant  receives  the  coal  from  a  number  of 
mines,  two  equalizing  units  are  required,  one  at  the  mine  between 
the  screening  plant  and  the  railroad  cars  and  the  other  between 
the  railroad  cars  and  the  washery.  If  sufficient  room  is  at  dis- 
posal and  a  regular  supply  of  railroad  cars  is  assured,  a  railroad 
yard  sufficiently  large  to  hold  enough  cars  to  load  out  at  least  an 
18-hr,  output  of  the  mine  will  be  a  more  economical  proposition 
than  a  large  bin  with  the  necessary  elevating  and  conveying  ma- 
chinery. But  with  this  arrangement  an  absolutely  regular  sup- 
ply of  railroad  cars  must  be  guaranteed.  This  is,  however, 
highly  problematical,  as  the  possibilities  of  such  an  ideal  supply 
of  railroad  cars  are  remote. 

To  arrive  at  the  proper  arrangement  of  the  different  units  it 
is  necessary  to  form  a  correct  idea  of  the  most  advantageous  loca- 
tion and  elevation  between  the  screening  plant  and  the  washery. 
The  following  facts  must  be  considered: 

The  elevation  of  the  screening  plant  is  fixed  by  the  elevation 
of  the  railroad  track  in  the  loading  yard  under  the  tipple.  To 
load  the  coal  directly  from  the  screens  into  the  railroad  cars  in 
the  most  efficient  manner  the  coal  should  neither  be  elevated  by 
separate  machinery  nor  should  it  drop  from  too  high  a  point. 
The  elevation  of  the  rails  under  the  tipple  are  the  base  from 
which  the  elevation  of  the  screening  plant  must  be  determined. 
In  a  screening  plant  the  use  of  elevators  can  be  easily  avoided, 
but  in  a  washery  conditions  are  different.  The  coal  must  pass 
over  a  number  of  different  pieces  of  apparatus  and  elevators 
cannot  be  entirely  avoided. 

It  has  been  previously  stated  that  modern  washeries.  are  de- 


UNITS  BETWEEN  SCREENING  PLANT  AND  WASHERY       87 

signed  with  the  point  in  view  of  avoiding  as  far  as  possible  the 
use  of  numerous  small  elevators  and  to  place  the  first  units  of  a 
washery  at  such  a  height  that  the  materials  can  be  conveyed 
either  by  gravity  or  in  sluiceways  with  water  through  the  whole 
plant.  On  account  of  this  method  of  construction  it  becomes 
necessary  to  elevate  the  raw  coal  from  the  screening  plant  to 
the  washery  at  a  considerable  height. 

Because  the  raw  coal  travels  from  the  screening  plant  to  the 
washery  by  way  of  the  raw-coal  bin  it  will  be  necessary  to  deter- 
mine how  this  bin  can  be  located  to  the  best  advantage  and  so  as 
to  make  the  carrying  of  the  coal  as  economical  and  efficient  as 
possible.  A  raw-coal  bin  of  even  a  few  hundred  tons'  capacity 
requires  a  heavy  supporting  structure  and  therefore  it  should  not 
be  located  any  higher  than  absolutely  necessary.  The  conclusion 
is  thus  reached  that  the  heaviest  elevating  should  not  be  done 
between  the  screening  plant  and  the  bin  but  between  the  bin 
and  the  washery. 

Some  designs  do  not  show  a  proper  bin  at  all  but  only  an  ar- 
rangement to  deposit  the  raw  coal  on  the  ground,  with  maybe 
a  low  retaining  wall  on  both  sides  to  keep  the  coal  from  spreading 
too  far.  These  walls  can  also  be  used  to  support  the  loading 
convej^ors.  The  coal  is  recovered  from  this  pile  by  a  conveyor 
running  in  an  underground  tunnel. 

For  the  elevation  of  great  quantities  and  the  required  uniform 
delivery  steep  bucket  elevators  have  proved  themselves  especially 
well  adapted.  Skip  hoists  cannot  be  considered,  because  the  de- 
livery is  irregular  and  intermittent.  For  these  reasons  it  is  ad- 
vantageous to  locate  the  raw-coal  bin  close  up  against  the  washery 
at  the  ground  level.  If  we  now  consider  that  the  coal  is  delivered 
from  the  screens  at  a  slight  elevation  above  the  track  level,  and 
that  the  top  of  the  raw-coal  bin,  on  account  of  its  required  large 
capacity,  is  at  a  much  higher  elevation,  a  normal  arrangement  of 
the  units  between  the  screening  plant  and  the  washery  will  be  as 
follows:  Elevating  or  conveying  machinery  to  overcome  slight 
differences  in  height  between  the  screening  plant  and  the  raw- 
coal  storage  bin  and  elevators  to  overcome  a  considerable  differ- 
ence in  height  for  feeding  the  washery  from  this  bin. 

For  the  conveyance  of  coal  to  the  raw  coal  bin  scraper  and 
belt  conveyors  are  admirably  adapted. 


88 


COAL  WASHING 


Scraper  conveyors  are  well  adapted  for  steep  grades  and 
short  distances.  Their  construction  is  simple  and  they  have 
large  capacity  with  relatively  small  power  consumption.  Belt 
conveyors  are  also  advisable  because  they  carry  the  smaller  sizes 
of  screened  coal  efficiently.  The  question  to  be  solved  is,  which 
of  these  two  types  of  conveyors  fulfills  the  conditions  to  the  best 
advantage? 

Belt  conveyors  are  theoretically  well  -adapted  for  this  purpose. 
The  delivery  chutes  from  the  screens  deposit  the  coal  upon  the 
belts  in  the  best  possible  manner.  The  hoppers  under  the 
screens  act  as  equalizers  and  permit  uniform  loading  of  the 
belts.  The  discharge  of  coal  from  the  belt  into  the  bin  is  carried 
on  easily  and  the  coal  is  transported  without  degradation.  But 
on  the  other  hand  belt  conveyors  are  rather  expensive  and  the 
belts  require  frequent  renewals. 

The  nfain  item  of  expense  in  the  upkeep  of  belt  conveyors  is 
that  of  the  renewal  of  the  belt.  With  ordinary  care  and  atten- 
tion a  good  grade  of  conveyor  belt  should  last  from  two  to  five 
years.  The  belts  may  have  a  speed  of  from  200  to  600  ft.  per 
minute  and  the  capacity  depends  upon  the  width  and  speed. 
The  following  table  gives  capacities  of  conveyors  usually  em- 
ployed. 

CAPACITIES  OF  TROUGHED  BELT  CONVEYORS 
Material — Coal,  weight  50  Ib.  per  cubic  foot 


Speed  200  ft. 

Speed  400  ft. 

Speed  600  ft. 

per  minute. 

per  minute. 

per  minute. 

Width 

Largest  size 
of  cube 

Tons 

Largest  size 
of  cube 

Tons 

Largest  size 
of  cube 

Tons 

of 
conveyor 

V.-IA 

which  can 
be  carried. 

per 
hour. 

which  can 
be  carried. 

per 
hour. 

which  can 
be  carried. 

per 
hour. 

belt. 

Inches. 

Inch. 

Inch. 

12 

2 

6 

A 

16 

1 

22 

16 

3 

16 

1~ 

34 

1 

50 

18 

4 

20 

H 

45 

1 

70 

20 

5 

30 

2 

60 

1 

100 

24 

6 

50 

3 

100 

1 

190 

30 

7 

100 

4 

200 

2 

360 

36 

9 

180 

6 

340 

2 

600 

TABLE  7 


Fig.  37  shows  a  scraper  conveyor  and  Fig.  38  a  belt  conveyor 
installation. 

Scraper  conveyors   are  comparatively  cheap,   quite   durable, 


UNITS  BETWEEN  SCREENING  PLANT  AND  WASHERY        89 

and  the  separate  parts  can  be  easily  and  cheaply  replaced  so  that 
they  are  better  adapted  for  rough  treatment.  But  they  grind  up 
the  coal  so  that  the  choice  between  the  two  types  of  conveyors 
depends  a  great  deal  upon  the  nature  and  size  of  the  material  to 
be  conveyed.  Soft  friable  coals  require  belt  conveyors,  espe- 
cially if  the  small  sizes  are  of  little  value.  To  a  hard  coal  that 
does  not  shatter  much  or  to  a  coking  coal  where  fines  are  not 


Fig.  37.     Scraper 'Convey or 

objectionable,  scraper  conveyors  will  be  better  adapted.  But 
the  scraper  conveyors  use  more  power  than  the  belt  conveyors 
so  that  the  cost  of  energy  is  another  consideration  that  must  be 
taken  into  account  in  making  the  proper  selection  of  the  most 
economical  type  of  conveyor. 

Scraper  or  belt  conveyors  are  not  only  used  for  the  filling  of 
the  raw-coal  bin  but  are  also  employed  for  the  conveyance  of  the 
different  materials  through  the  washery,  and  especially  for  the 


90 


COAL  WASHING 


filling  of  the  washed  coal  bins,  which  are  usually  arranged  in  a 
series  of  separate  bins  set  in  a  row.  To  fill  these  bins  by  means 
of  scraper  conveyors  it  is  only  necessary  to  put  in  openings  pro- 
vided with  sliding  gates  in  the  bottom  of  the  conveyor  trough, 
at  the  proper  points. 

Belt  conveyors  can  be  easily  equipped  with  trippers,  which 
will  permit  the  discharge  of  the  coal  at  any  point,  or  fixed  dis- 
charge stations  can  be  built  into  the  conveyor.  The  trippers 
can  be  either  hand-operated  or  they  can  be  made  automatic  and 


Fig.  38.    Belt  Conveyor 

self -reversing.  By  locating  movable  stops  on  the  track  rails  the 
coal  can  be  discharged  either  over  the  whole  length  of  the  bin 
or  over  any  part  of  it.  For  bins  which  are  located  at  right 
angles  to  the  general  flow  of  material,  shuttle  conveyors  can  also 
be  used  to  advantage. 

Which  one  of  the  arrangements  should  be  employed  depends 
entirely  on  the  type  of  the  bin.  Generally  speaking,  belt  con- 
veyors on  account  of  their  light  weight  and  their  adaptability 
to  almost  any  condition  are  to  be  used  for  long  distances  and  for 
fine  coal  if  it  is  not  too  wet.  For  coarse  and  wet  coal  and  for 
short  distances  scraper  conveyors  are  to  be  preferred.  For 


r\/T8  BETWEEN  SCREENING  PLANT  AND  WASHERY        91 

short  horizontal  conveying,  shaking  conveyors  of  the  Marcus  or 
Zimmer  type  can  also  be  used  to  great  advantage. 

The  Design  and  Construction  of  Raw  Coal  Bins.  If  the  raw- 
coal  bin  is  included  within  the  main  washery  building  the  same 
material  is  used  in  the  construction  of  both  units.  This  is  either 
timber,  steel  or  reinforced  concrete,  or  in  some  rare  cases,  cast 
iron.  Raw-coal  bins  are  built  either  rectangular  or  round. 


Fig.  39.     Tripper  for  Belt  Conveyor 


Rectangular  bins  are  more  expensive  in  construction  but  have  a 
greater  capacity  per  square  foot  of  ground  space  occupied  than 
have  circular  bins.  Rectangular  bins  permit  also  a  more  perfect 
arrangement  for  loading  and  unloading  apparatus  than  do  round 
ones,  especially  if  the  necessary  capacity  increases. 

The  capacity  of  the  raw-coal  bin  depends  upon  the  output  of 
the  mine  and  should  be  at  least  as  large  as  the  daily  production. 
Bins  with  a  capacity  of  4,000  tons  have  been  constructed  and 
some  of  the  latest  designs  show  bins  with  6,000  tons  capacity. 


92 


COAL  WAS'HING 


Fig.  40  is  the  photograph  of  a  raw-coal  bin  of  4,000  tons'  ca- 
pacity, built  of  re-inforced  concrete,  and  Fig.  41  gives  the  gen- 
eral drawing  of  the  same  bin. 


Fig.  40.     4,000  Tons  Raw  Coal  Bin,  built  of  Re-inforced  Concrete 


B.     FEEDING  THE  WASHER Y  FROM  THE  KAW  COAL  BIN 

Requirements.  The  mechanical  appliances  for  supplying  the 
washery  with  raw  coal  are  of  the  greatest  importance.  The  con- 
tinuous and  regular  operation  of  the  washery  depends  entirely 
on  a  steady  coal  supply ;  therefore,  the  strictest  specifications 
should  be  applied  for  the  design  and  construction  of  these  appli- 
ances. 

The  capacity  must  be  fully  equal  to  that  of  the  washery.  It 
must  be  considered  that  elevators  handling  large  tonnages  re- 
quire large,  heavy  buckets.  On  account  of  the  size  of  the  weights 
to  be  hoisted  and  the  height  to  which  the  coal  must  be  elevated, 
failures  of  the  chain  are  disastrous  and  repairs  become  difficult 
and  take  up  much  time.  For  this  reason  it  is  preferred  to  use 
in  large  plants  two  elevators. 

On  the  other  hand  the  general  arrangement  and  the  space 
available  permit  the  installation  of  only  one  raw-coal  bin.  This 
makes  the  installation  of  more  than  two  elevators  almost  impos- 
sible on  account  of  the  lack  of  room  at  the  bottom  of  the  bins. 
The  conditions  at  the  discharge  end  of  the  elevators  are  similar. 

As  a  usual  thing  only  one  unit  is  provided  to  receive  the  raw 
coal,  and  economic  reasons  demand  the  concentration  of  the  flow 
of  coal  through  the  washery.  Therefore  we  must  seek  absolutely 


UMTti  BETU'EEX  SCREENING  PLANT  AND  WASHERY       93 


94  COAL  WASHING 

reliable  operation  of  the  elevators.  This  can  only  be  obtained 
by  using  the  best  of  materials,  by  ample  dimensions  of  the  carry- 
ing members  and  by  the  best  workmanship.  First  cost  cannot 
be  considered  if  we  take  into  account  the  increase  in  cost  of 
operation  caused  by  repairs  and  interruptions. 

But  even  with  all  these  precautions  it  is  advisable,  whenever 
possible,  to  have  a  complete  spare  elevator  on  hand.  In  case  of  a 
wreck  or  the  collapse  of  an  elevator  repairs  take  a  long  time,  and 
for  this  reason  a  spare  elevator  ought  to  be  in  place  for  imme- 
diate use. 

If  these  precautions  are  required  for  continuous  operation, 
there  remains  still  the  necessity  for  a  regular  and  uniform  sup- 
ply. This  is  of  the  utmost  importance  for  the  efficient  working 
of  a  washery.  On  account  of  the  existing  conditions,  which  re- 
quire the  elevating  of  coarse-grained  material  at  a  steep  angle  to 
a  considerable  height,  absolutely  continuous  discharge  can  hardly 
be  accomplished.  But  the  continuous  bucket  elevator  comes 
nearer  to  fulfilling  the  conditions  than  any  other  apparatus. 

Bucket  Elevators.  Bucket  elevators  are  used  in  coal  wash- 
eries  for  various  purposes.  The  most  important  of  these  are  the 
following:  Supplying  the  washery  with  raw  coal;  dewatering 
the  washed  coal  and  elevating  it  into  the  washed  coal  bins;  sup- 
plying the  rewash  jigs;  dewatering  the  sludge;  dewatering  the 
secondary  product  (boiler  house  coal)  and  elevating  it  to  the 
boiler  house  coal  bin ;  dewatering  the  refuse  and  elevating  it  into 
the  refuse  bin. 

The  design  and  construction  of  elevators  depend  upon  the 
material  to  be  handled.  To  avoid  repetitions  the  main  types  of 
elevators  used  for  the  different  purposes  will  be  described  under 
a  common  head. 

Most  of  the  elevators  are  constructed  with  a  two-strand  steel- 
link  chain.  The  chain  links  and  the  connecting  bolts  and  rods 
ought  to  be  fabricated  out  of  the  best  medium  carbon  machinery 
steel.  Small  elevators  have  flat  links,  but  for  heavy  elevators 
links  with  upset  ends  are  to  be  preferred.  Under  ordinary  con- 
ditions and  for  dewatering  elevators,  buckets  are  fastened  to  the 
chain  at  every  other  link.  For  raw-coal  elevators  buckets  are 
provided  at  every  link,  making  it  a  continuous  bucket  elevator. 

Fig,  42  gives  the  working  drawing  of  a  continuous  bucket  ele- 


UNITS  BETWEEN  SCREENING  PLANT  AND  WASHERY        95 


96 


COAL  WASHING 


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UNITS  BETWEEN  SCREENING  PLANT  AND  WASHERY       97 


vator  for  raw  coal  and  Fig.  43  the  details  for  the  bucket  and 
the  chain.     In  Fig.  44  a  bucket  with  perforated  sides,  as  used 


Fig.  44.     Washed  Coal  Elevator  Bucket 

for  washed  coal  and  refuse,  is  shown,  while  Fig.  45  shows  buckets 
and  chains  of  a  ' '  Liihrig ' '  dewatering  elevator. 

Buckets  are  made  from  steel  plates,  securely  riveted  to  the  end 
plates  and  provided  at  the  top   edges  with  stiffening  bands. 


-is 


F 


UK- 


Fig.  45.     Bucket  and  Chain  for  Liihrig  Dewatering  Elevator 

Wide  buckets  have  also  a  stiffener  in  the  middle  to  prevent  bulg- 
ing out  of  the  sides.     The  ends  of  the  buckets  are  fastened  to  the 


98 


COAL  WASHING 


sides  with  angle  iron  or,  in  some  instances,  the  ends  are  made 
of  malleable  iron.  Malleable-iron  buckets  or  those  pressed  from 
one  piece  of  steel  plate  are  seldom  used.  Dewatering  elevators 
have  buckets  with  sides  and  ends  made  out  of  perforated  plates. 
The  size  of  the  perforation  must  be  in  proportion  to  the  size  of 
the  coal  handled. 

The  elevator  chains  at  the  top  are  carried  over  sprocket  wheels 
or  head  drums.  On  the  lower  end  foot  drums  are  universally 
used.  Square,  hexagon  and  octagon  drums  are  employed,  de- 


Fig.  46.     Octagon  Head-drum  for  Liilirig  Elevator 


pending  on  the  pitch  of  the  chain  links.  Octagon  drums  can  be 
used  with  short  pitch  chain,  whereas  those  of  long  pitch  require 
square  drums.  The  pitch  of  the  chain  links  varies  from  8  to  18 
inches. 

An  octagon  head  drum  with  discharge  plates  is  shown  in 
Fig.  46,  and  Fig.  47  shows  the  upper  part  of  a  washed  coal  ele- 
vator. 

The  take-ups  are  generally  placed  at  the  foot  of  the  elevator 
and  only  on  small  elevators  are  the  take-ups  placed  at  the  top. 
Head  and  foot  drums  are  either  made  of  steel  or  have  steel  wear- 
ing plates.  The  chains  are  guided  between  angle  irons  which 
ought  to  be  protected  by  wearing  plates. 

Fig.  49  is  an  illustration  of  a  single  take-up.     For  washed  coal 


UNITS  BETWEEN  SCREENING  PLANT  AND  WASHERY        99 

and  refuse  elevators,  where  the  take-up  is  under  water,  the 
take-up  screw  and  its  nut  are  made  of  bronze. 

Elevators  are  usually  driven  through  a  train  of  gears  or  by  a 
sprocket  or  silent  chain  drive.  Belt  drives  should  not  be  used 
on  account  of  the  daifger  of  slipping.  The  gear  wheels  should 
be  of  steel  while  for  heavy  elevators  manganese  steel  gears  ought 
to  be  specified. 

To  prevent  the  running  back  of  an  elevator,  when  the  power 


Eig.  47.     Head  of  Washed  Coal  De*watering  Elevator 

goes  off  or  when  the  belt  slips,  various  forms  of  hold-backs  are 
made.  The  simplest  construction  uses  a  pawl  and  ratchet  wheel. 
Another  type  illustrated  in  Fig.  50  has  the  pawl  mounted  on  a 
split  sleeve,  which  is  clamped  on  the  hub  of  the  driving  pinion 
by  means  of  four  bolts,  fitted  with  compression  springs.  These 
springs  provide  sufficient  friction  to  make  the  pawl  tend  to  re- 
volve with  the  pinion.  A  stud  projecting  from  the  pawl  strikes 
a  stop  and  prevents  its  turning.  The  instant  the  power  goes 
off,  however,  and  the  pinion  starts  to  reverse,  the  pawl  turning 
back  with  the  pinion  throws  its  tooth  into  the  teeth  of  the  gears 


100 


COAL  WASHING 


and  thereby  locks  the  entire  mechanism.  Both  of  the  above 
hold-backs  have  the  disadvantage  that  the  motion  of  the  elevator 
is  arrested  suddenly. 

The   roller   hold-back  illustrated   in   Fig.   51   acts  somewhat 


Fig.  48.     Washed  Coal  Elevator 

smoother.  It  is  designed  on  the  same  principle  as  a  coaster 
brake  of  a  bicycle. 

A  still  better  type  of  hold-back  uses  a  band  brake  which  is 
loose  as  long  as  the  elevator  moves  in  the  right  direction,  but 
as  soon  as  it  commences  to  run  backwards,  this  brake  is  tightened 
and  stops  thereby  the  reverse  motion  gradually  and  effectively. 

The  framework  supporting  the  chains  may  be  made  of  steel 


UNITS  BETWEEN  SCREENING  PLANT-  '^Nfr  WlLSHERY  '/101 


for  the  raw-coal  elevators,  but  for  dewaterjig'  iejevaloi^  ifiirfobi: 
frames  are  to  be  preferred.  The  head  and  jack  shafts  ought  not 
to  be  supported  on  the  elevator  frames,  but  on  a  separate  sup- 
porting structure.  This  is  advisable  in  order  to  facilitate  re- 


Fig.  49.     Single  Take-up  for  Coal  Elevator 

pairs  and  to  keep  the  gears  in  better  alignment.  The  size  of  the 
bucket  elevators  varies  considerably.  The  elevators  for  heavy 
tonnage  are  massive  and  heavy,  and  must  therefore  run  slowly  in 
order  to  prevent  rapid  wear.  Dewatering  elevators  also  must 


Fig.  50.     Hold-back  for  Elevator 

operate  at  a  slow  speed  to  permit  the  draining  out  of  the  water 
before  the  buckets  reach  the  discharge  point. 

Dewatering  elevators  must  be  inclined  to  such  a  degree  that 
the  water  running  out  of  one  bucket  will  not  drip  into  the  one 


102 


COAL  WASHING 


next  lowfer. . ,  K^w-CQaf  elevators  could  be  installed  vertically,  but 
this  arrangement  would  require  too  much  room  at  the  head  as 
well  as  at  the  foot  end.  The  inclination  of  a  raw-coal  elevator 
depends  entirely  upon  the  room  available.  The  speed  of  raw- 


Fig.  51.     Hold-back  for  Elevator 

coal  elevators  ought  not  to  exceed  100  ft.  per  minute  while  de- 
watering  elevators  should  not  be  run  faster  than  from  50  to  60  ft. 
per  minute.  The  maximum  capacity  of  elevators  in  coal  wash- 
eries  is  about  200  tons  per  hour. 

TABLE  GIVING  GENERAL  DIMENSIONS  AND  CAPACITIES  OF  RAW  COAL  AND 
WASHED  COAL  ELEVATORS 


Height  in  feet 

Raw  Coal 
Elevators 

30-135 

Washed  Coal 
Elevators 

45-120 

Width  in   inches 

12-  50 

18-  60 

Number  of  buckets 

25-200 

30-100 

Inclination   in  degrees 

40-  80 

40-  60 

Capacity  in  tons  per  hour  

25-250 

15-100 

H.P.  required    
Dewatering  effect  in   percentage   of 
water  remaining  in  the  coal 

5-  70 

\ 

12-  35 
13-  10 

TABLE  8 


CHAPTER  XIII 
CLASSIFYING  OF  THE  FINE  COAL 

The  coal  discharged  from  the  raw-coal  elevator  at  the  highest 
point  of  the  washery  must  be  separated  from  its  impurities  and 
screened  into  the  different  sizes  demanded  by  consumers.  The 
ways  and  means  employed  in  this  process  vary  according  to  the 
system  of  washing  followed.  As  all  the  bituminous  coals  are 
similar  in  nature,  it  becomes  necessary  to  study  the  different 
points  involved  before  we  can  follow  clearly  the  development  of 
the  different  methods  used. 

The  first  question  is:  Shall  the  raw  coal  as  a  whole  be  sub- 
jected to  washing  or  will  it  be  better  to  screen  out  the  fines? 
This  main  question  can  be  divided  into  several  parts,  because 
different  conditions  require  different  solutions.  The  first  sub- 
question  is:  Is  it  possible  to  improve  the  fines  by  washing? 
This  can  be  answered  in  the  negative  with  all  possible  assurance. 
The  consideration,  however,  as  to  how  fine  the  coal  can  be  to  be 
successfully  washed  is  still  disputable. 

Laboratory  investigations  and  results  of  actual  operation  give 
largely  different  limits.  It  can  safely  be  stated,  however,  that 
coal  passing  through  a  20-mesh  screen  will  not  show  any  marked 
improvement  by  washing.  However,  it  is  not  possible  to  give  an 
absolutely  binding  limit  for  all  sorts  of  coal.  A  correct  decision 
in  regard  to  the  permissible  fineness  of  the  coal  to  be  washed  can 
only  be  arrived  at  through  accurate  and  reliable  tests. 

Thomas  James  Drakeley  made  some  tests  to  determine  whether 
any  advantage  would  accrue  from  removal  of  the  dust  from  the 
raw  coal  previous  to  washing,  which  gave  the  following  results: 
The  sample  of  raw  coal  was  screened  over  a  series  of  screens 
having  30,  60,  90  and  120  meshes  to  the  inch,  respectively.  The 
ash  content  of  the  powders  were  determined  and  compared  with 
the  ash  content  of  the  corresponding  dried  slime.  In  all  cases 

103 


104 


COAL  WASHING 


the  fine  coal  removed  from  the  raw  coal  yielded  less  ash  than  the 
settlings.     An  example  L  given  in  the  following  table : 

ASH  CONTENT  OF  RAW-COAL  DUST  AND  SLIME 

Slime  from  Washed  Coal 


Through 


Raw  Coal 
Powder  screened 


Over 


Percentage 
of  Ash 


The  whole  of  the  slime 

passed  through  a  sieve 

with  120  meshes  to 

the  inch. 


30  mesh 
60  mesh 
90  mesh 
120  mesh 

60  mesh 
90  mesh 
120  mesh 

23.09 
22.48 
20.50 
17.71 

Percentage  of  Ash 
26.35 

TABLE  9 

It  is  obvious,  therefore,  that  during  the  washing  process  some 
of  the  impurities  disintegrate  and  pass  away  with  the  finest  coal. 
The  settlings  are  in  consequence  inferior  to  the  dry  coal-dust. 
Hence,  it  would  appear  to  be  economical  to  remove  the  dust  pre- 
vious to  washing.  This  dust  could  be  mixed  with  the  washed 
coal  without  unduly  diminishing  its  value.  The  settlings  would 
be  composed  of  a  larger  proportion  of  ash-yielding  constituents 
and  could  be  regarded  justly  as  refuse.1 

If  it  has  been  proved  that  the  fines  can  not  be  improved  by 
washing,  the  next  question  is:  Is  it  possible  to  screen  out  the 
fines  in  the  dry  state?  The  condition  of  the  coal  in  the  mine 
will  primarily  determine  this  question.  If  it  comes  from  a  so- 
called  dry  mine,  dry  screening  is  probably  possible,  but  if  the 
coal  comes  out  of  a  wet  mine,  or  contains  more  than  from  5  to  6 
per  cent,  of  moisture,  all  efforts  at  dry  screening  will  fail. 

The  third  question  is:  Are  the  advantages  gained  from  the 
screening  out  of  the  fines  sufficient  to  justify  the  cost  of  installa- 
tion and  operation  of  the  necessary  machinery?  The  considera- 
tions governing  this  point  are  as  follows:  (a)  If  coal  is  to  be 
sized  before  washing,  the  screening  operation  will  without  doubt 
be  more  perfect,  (b)  Dust  mixed  Avith  water  becomes  slime, 
which  hinders  the  jigging  process.  If  the  dust  has  been  elimi- 
nated, the  jigs  will  deliver  a  refuse  more  nearly  free  from  good 
coal  and  therefore  work  with  greater  efficiency,  (c)  If  the  im- 


ley. 


From  "Coal-Washing  :     A  Scientific  Study,"  by  Thomas  James  Drake- 


CLASSIFYING  OF  FINE  COAL  105 

purities  in  the  raw  coal  contain  fireclay,  which  dissolves  freely  in 
the  water,  and  on  the  other  hand  the  coal  dust  is  comparatively 
clean,  the  fireclay  and  the  dust  will  be  mixed  together  in  the  jigs 
and  the  resulting  fines  will  be  high  in  ash.  The  fines  in  this 
case  will  be  of  better  quality  if  screened  out  before  washing. 
(d)  If  the  fines  themselves  contain  firecla3r,  the  wash  water  will 
become  thick  and  difficult  to  clarify,  requiring  large  and  expen- 
sive clearing  basins,  (e)  If  the  dust  is  sufficiently  low  in  ash 
it  can  be  mixed  with  the  washed  coal  without  increasing  the  ash 
in  the  final  product.  This  method  will  also  facilitate  the  diffi- 
cult problem  of  de watering  the  fines,  (f)  The  possibility  of  re- 
ducing the  ash  in  the  washed  product  without  increasing  the 
loss  of  good  coal,  through  screening  out  the  dust,  permits  the 
addition  of  dust  higher  in  ash  than  would  be  possible  if  the  dust 
were  not  thus  screened  out. 

In  the  light  of  the  foregoing  considerations,  if  heed  is  given 
to  the  fact  that  the  removal  of  the  fines  is  comparatively  simple, 
the  question  of  dust  separation  can  be  answered  as  follows: 
Except  in  cases  where  the  raw  coal  contains  too  much  moisture 
or  where  the  nature  of  the  material  is  such  that  little  dust  or 
sludge  is  produced,  the  installation  of  a  dust  separator  ahead  of 
the  jigs  is  to  be  recommended. 

The  second  question,  in  regard  to  the  proper  method  of  wash- 
ing is :  Shall  the  coal  be  sized  before  or  after  washing  ?  Close 
sizing  before  washing  is  not  necessary.  Only  with  a  coal  that  is 
difficult  to  wash  and  one  that  is  at  the  same  time  hard  and  not 
liable  to  make  a  great  amount  of  fines,  is  sizing  before  washing 
to  be  preferred.  Otherwise  the  sequence  of  operations  will  de- 
pend upon  the  nature  of  the  coal — that  is,  how  closely  it  should 
be  sized  or  whether  it  should  be  washed  unsized.  Therefore,  in 
washing  plants  we  find  at  present  the  following  main  methods  of 
procedure:  (1)  Sizing  before  washing,  and  the  employment  of 
separate  jigs  for  each  size.  (2)  Preparatory  separation  into  two 
or  three  grades  in  addition  to  dust,  and  separate  jigs  for  each  of 
the  three  or  four  sizes.  (3)  Sizing  into  coarse  and  fine  coal 
only,  and  separate  jigs  for  both  sizes.  (4)  Jigging  of  the  un- 
sized raw  material  with  subsequent  sizing  into  coarse  and  fine 
coal  and  rewashing  of  the  fines.  (5)  Washing  of  the  unsized 
raw  coal  without  any  sizing  or  rewashing. 


106  COAL  WASHING 

With  types  2  to  5,  the  final  sizing  of  the  coal  for  market  is 
performed  after  washing.  Type  5  is  used  for  coking  coals,  and 
then  only  if  the  coal  is  easily  washed. 

Considering  that  not  every  washery  is  provided  with  a  dust- 
collecting  plant,  and  that  the  coal  is  sized  either  before  or  after 
washing,  the  required  apparatus  may  be  classified  in  the  follow- 
ing order:  Dust  separating  and  collecting  machinery,  sizing 
machinery,  washing  machinery. 

Dust  Removal.  To  separate  the  dust  at  once  from  the  total 
raw  coal  would  give  only  imperfect  results.  Therefore,  it  is 
desirable  to  screen  the  coal  at  first  into  two  sizes  besides  the 
lump — a  coarse  product  from  %  in.  to  3  in.,  and  a  fine  coal 
from  %  in.  to  dust. 

A  plant  for  the  separation  of  dust  should  screen  out  in  as  per- 
fect a  manner  as  possible  all  the  dust  up  to  a  previously  deter- 
mined size  without  carrying  away  particles  of  coarser  coal;  be- 
cause, if  part  of  the  dust  remains  in  the  coarse  coal,  the  intention 
of  facilitating  the  jigging  and  avoiding  cumbersome  settling 
basins  will  not  have  been  fulfilled.  On  the  other  hand,  if  coarse 
coal  goes  with  the  dust  some  of  the  material  which  can  be  im- 
proved by  jigging  will  not  get  the  benefit  of  this  improvement. 
Therefore,  the  fundamental  principles  are  as  follows:  The  util- 
ity of  a  plant  for  screening  out  the  dust  is  in  direct  proportion 
to  its  perfect  removal  of  dust  containing  no  coarse  coal. 

Dust  can  be  removed  either  on  screens  or  by  means  of  an  air 
current.  Screening  appears  theoretically  preferable  because  the 
size  of  the  perforations  in  the  screen  plates  determines  the  largest 
size  of  particles  that  will  pass  through  them.  Failures  encoun- 
tered with  the  earlier  installations  were  caused  by  the  difficulty 
in  keeping  the  perforations  clear.  This  trouble,  however,  was 
overcome  by  the  use  of  vibrating  screens. 

One  great  disadvantage  of  a  screening  plant  for  dust  removal 
lies  in  the  fact  that  such  a  plant  is  noisy,  subject  to  frequent 
repairs,  and  that  any  air-tight  inclosure  for  it  is  expensive  and 
prevents  the  inspection  of  the  moving  parts.  A  dust  separator 
using  air  currents  works  noiselessly  -and  has  no  moving  parts 
except  the  exhaust  fan.  It  is  also  easy  to  inclose  this  type  of 
apparatus  in  an  air-tight  casing,  which  is  not  objectionable 
because  no  moving  parts  requiring  repairs  are  inclosed.  With 


CLASSIFYING  OF  FINE  COAL 


107 


an  air  separator,  however,  it  is  not  possible  to  get  perfect  separa- 
tion in  regard  to  sizes;  some  coarse  material  is  always  carried 
away  with  the  dust  or  some  dust  remains  in  the  coarse  coal.  In 
addition  to  a  careful  selection  of  the  apparatus,  its  constant  con- 


Fig.  52.     Vibrating  Screen 

trol  and  adjustment  are  required  to  insure  satisfactory  results. 

Fig.  52  shows  a  vibrating  screen  as  used  in  Europe  for  the 
screening  out  of  dust  from  coal. 

The  screen  frames  are  fastened  with  wedges  in  an  inclined 


108 


COAL  WASHING 


box  ' '  a. "  The  screen  itself  is  a  fine  mesh  brass-wire  cloth.  The 
vibrations  are  caused  by  a  shaft  "b"  having-  multiple  cams  and 
are  uniformly  transmitted  over  the  whole  screen  by  wooden 
springs  "h." 

The  coal  passes  through  the  chute  "Q"  and  over  the  hog-back 
"d-d"  on  the  screens.  The  dust  is  collected  in  the  hopper 
"e-e"  and  can  be  carried  away  by  the  belt  conveyor  "f. "  The 
screens  are  inclosed  in  a  dust  tight  casing.  This  apparatus  can 


Fig.  53.     Dust  Collecting  Plant.     Elevation 

remove  dust  finer  than  %2  in.  if  the  coal  contains  from  4  to  5 
per  cent.,  and  dust  finer  than  Vs2  in.  if  the  coal  contains  from 
2  to  2^  per  cent,  of  moisture.  Screens  30  in.  wide  by  5  ft.  long 
can  handle  from  2  to  4  tons  of  coal  per  hour.  This  small  capac- 
ity would  require  a  great  number  of  screens  to  handle  the  quanti- 
ties necessary  for  a  modern  coal  washer. 

A  dust  collecting  plant  in  connection  with  a  sizing  screen  is 
illustrated  in  Figs.  53  and  54. 

The  elevator  "1"  discharges  the  coal  over  the  chute  "2"  on 


CLASSIFYING  OF  FINE  COAL 


109 


the  screen  "3,"  with  %  in.  round  perforations.  The  oversize 
goes  to  the  sluiceway  "4"  and  the  undersize  into  the  hopper 
''5."  The  revolving  feeder  "6"  carries  the  fine  coal  in  a  uni- 
form stream  across  the  slot  "7."  A  current  of  air,  coming 


Fig.  54.     Dust  Collecting  Plant.     Plan  View 

through  the  pipe  "14"  passes  through  the  coal.  The  coarser 
coal  drops  into  the  chute  "11"  and  the  dust  is  blown  into  the 
hopper  "8"  where  the  coarser  particles  are  deflected  by  a  baffle 
plate.  The  finer  dust  collects  against  the  inclined  wall  of  the 
hopper  and  can  be  discharged  through  a  gate  in  a  second  hopper, 
if  it  is  desirable  to  make  two  sizes  of  dust.  The  cleaned  air  re- 


Fig.  55.     Dust  Collecting  Plant 

turns  through  a  pipe  "10"  to  the  exhaust  fan  "9."     Fig.  55 
shows  a  photograph  of  a  similar  installation. 

The  Use  of  Dust.     The  use  to  which  the  dust  should  be  put 
is  determined  by  the  nature  of  the  coal,  the  amount  of  ash  and 


110  COAL  WASHIXG- 

the  method  of  collecting  the  dust.  "With  a  coking  coal,  dust  can 
be  mixed  with  the  washed  fuel  if  the  ash  content  will  permit. 
This  is  the  simplest  method  of  dust  disposal,  because  it  helps  in 
the  dewatering  of  the  washed  coal.  If  the  ash  content  is  high, 
at  least  a  portion  can  be  mixed  with  the  washed  coal. 

If  the  dust,  however,  is  excessively  high  in  ash  it  can  be  fur- 
ther treated  in  connection  with  the  sludge,  or  it  can  be  used  as 
fuel  under  a  boiler  or  in  cement  kilns,  or  it  can  be  made  into 
briquets.  With  fuel  coal,  the  dust  can  be  either  briquetted  or 
used  under  boilers ;  and  if  it  is  very  high  in  impurities  a  consid- 
erable amount  of  it  must  be  thrown  away. 

C.     SIZING  OP  THE  COAL 

Sizing  of  coal  can  be  divided  in  two  groups  of  processes: 
(1)  Sizing  as  a  preparatory  process  for  the  jig  work,  and  (2) 
sizing  as  a  subsequent  process  after  jigging  to  produce  the 
proper  grades  demanded  by  the  market.  In  both  instances  the 
same  types  of  screens  can  be  used  and  the  difference  exists  only 
in  the  following  details : 

(a)  In  the  preparatory  sizing  fewer  screen  plates  with  differ- 
ent size  perforations  are  required  on  account  of  the  fewer  sizes 
demanded. 

(b)  The  two  processes  demand  a  different  arrangement  of  the 
screens.     In  the  first  case  the  screen  products  are  fed  to  the 
jigs  and  in  the  latter  case  the  different  sizes  are  discharged  di- 
rectly into  loading  bins. 

(c)  The  preparatory  sizing  is  a  dry  screening  process  only. 
The  sizing  after  washing  is  at  the  same  time  a  dewatering  process. 

Preparatory  Sizing1.  Preparatory  sizing  is  only  a  preliminary 
process;  the  final  sizing  takes  place  after  washing.  The  sizes 
made  in  preparatory  sizing  depend  upon  the  character  of  the 
impurities  and  upon  their  amount  in  each  size.  A  typical  ex- 
ample is  shown  below : 


No.  1 

No.  2 

No.  3 

No.  4 

No.  5 

1 

2 

3 

4 

5 

I.  Group      

H"-3" 

*"-3" 

§"_3" 

H"-3" 

1"_3" 

II.  Group 

1"_1" 

0-2" 

O-I" 

III.  Group          .... 

0_i"- 

0-4" 

O-I" 

TABLE  10 


CLASSIFYING  OF  FINE  COAL  111 

An  exact  sizing  is  not  necessary,  as  the  grading  of  sizes  is  only 
preliminary  and  within  wide  limits. 

Sizing  after  Washing.  The  arrangement  of  screens  depends 
upon  the  customary  sizes  demanded  by  the  market,  but  no  exact 
standards  have  been  thus  far  established.  In  Illinois  five  sizes 
are  customary,  but  no  two  washeries  are  producing  exactly  the 
same  sizes.  The  range  is  as  follows: 


No.  1  extra    No.  1    No.  2  extra 

No.  2 

No.  3      No.  4 

No.  5 

inches 

inches 

inches 

inches 

inches    inches 

inches 

Always 

under   .  . 

3| 

3£ 

2i 

2} 

H              I 

T/16 

Always 

over    .  .  . 

If 

U 

i 

i         94o 

0 

TABLE  11 

The  operators  of  Williamson  County,  Illinois,  have  agreed  to 
the  following  standard  sizes: 


No     1 

Through 
3"     round,  holes 

Over 
If"  round  holes 

No    2 

1-2"  round  holes 

1"     round  holes 

No    3 

1"     round-  holes 

4"  round  holes 

No    4 

\"  round  holes 

J"  round  holes 

No.   5 

i"  round  holes 

TABLE  12 

The  percentage  of  each  size  as  well  as  the  amount  of  refuse 
contained  therein  are  of  great  importance  when  considering  the 
arrangement  of  a  washery.  It  is  therefore  necessary  to  deter- 
mine these  percentages  in  advance,  since  they  form  the  basis 
upon  which  the  capacities  of  all  the  apparatus  employed  in  a 
washery,  such  as  screens,  jigs,  elevators  and  storage  bins  for  the 
washed  coal  and  the  refuse  must  be  calculated. 

This  percentage  of  the  different  sizes  does  not  remain  constant 
during  the  life  of  a  mine.  Small  variations  can  be  taken  care  of 
by  figuring  the  capacities  of  the  different  pieces  of  apparatus - 
as  well  as  that  of  the  bins  somewhat  larger  than  necessary.  If, 
however,  the  percentage  should  vary  considerably,  it  will  be 
necessary  to  change  the  size  of  each  group.  Suppose  that  the 
screened  coal  from  a  certain  mine  gives,  by  making  the  sizes 
shown  in  column  "2"  (see  Table  13),  the  percentages  found  in 
column  ''3."  On  account  of  a  change  in  the  character  of  the 


112  COAL  WASHING 

coal  these  percentages  are  altered  as  shown  in  column  "4."  If 
the  consumer  will  agree  to  it,  it  will  be  possible  to  get  the  same 
percentages  for  each  size  by  changing  the  perforations  in  the 
screens  as  shown  in  column  5  (see  following  table). 


1 

2 

3 

4 

5 

Percent- 

Percent- 

Group 

Size  I 

age  of 
Size  I 

age  of 
Size  II 

Size  II 

Per  cent. 

Per  cent. 

No.   1    . 

3"  to  2" 

18 

14 

3"  to  If" 

No.   2    

2"  to  U" 

13 

12 

If"  to  1" 

No.  3    

li"  to  f  " 

18 

16 

1"  to  3V 

No    4 

|"  to  |" 

18 

20 

%e"  to  %6" 

No.   5    

1"  to  0 

33 

38 

%e"  to  0 

TABLE  13 


The  importance  of  knowing  the  percentage  of  the  different 
sizes  for  a  correct  installation  of  the  different  units  in  a  washery 
can  be  shown  in  the  following  tables: 


Lump 

Egg 

l 

2 

3 

4 

5 

Dust 

Total 

Coal 

Per 

Per 

Per 

Per 

Per 

Per 

Per 

Per 

Per 

cent. 

cent. 

cent. 

cent. 

cent. 

cent. 

cent. 

cent. 

cent. 

I  

24 

. 

5 

6 

7 

8 

32 

18 

100 

II.      ... 

24 

9 

7 

12 

11 

9 

28 

. 

100 

III.      .  . 

9 

13.5 

7.9 

19.5 

50.1 

100 

IV.      .. 

5 

7.5 

7.4 

7.5 

12.5 

60.1 

100 

TABLE  14 

For  determining  the  size  and  number  of  jigs  and  the  size  of 
the  bins  the  figures  in  Table  14  are  not  sufficient.  The  amount 
of  impurities  in  each  size  and  the  yield  of  washed  coal  play  an 
important  part  in  the  proper  selection  of  the  number  of  jigs  and 
size  of  bins.  Table  15  shows  the  percentage  of  coal  I.  in 
Table  14. 


Lump 
Per  cent. 

No.  1 
Per  cent. 

No.  2 
Per  cent. 

No.  3 
Per  cent. 

No.  4 
Per  cent. 

No.  5 
Per  cent. 

Dust 

Coal     .  .  . 

93 

92 

91 

88 

84 

80 

72 

Eefuse     . 

7 

8 

9 

12 

16 

20 

28 

Total     .  . 

100 

100 

100 

100 

100 

100 

100 

TABLE  15 


A  daily  capacity  of  3,000  tons  figured  on  the  above  basis  will 
give  the  following  quantities: 


CLASSIFYING  OF  FINE  COAL  113 


Lump 

No.  1 

No.  2 

No.  3 

No.  4 

No.  5 

Dust 

Total 

Total 

ra\v  coal    .  . 

.      720 

150 

180 

210 

240 

960 

540 

3,000 

Clean 

coal    

.      669.6 

138 

163.8 

184.8 

201.6 

768 

388.8 

2,514.6 

Refuse 

50.4 

12 

16.2 

25. 

2 

38.4 

192 

151.2 

485.4 

Total 

.      720 

150 

180 

210 

240 

960 

540 

3,000 

TABLE  16 

Tests  have  shown  that  the  yield  and  amount  of  impurities  in 
the  washed  coal  can  be  obtained  as  shown  in  the  first  and  second 
horizontal  lines  of  Table  17.  This  gives  with  a  capacity  of  3,000 
tons  the  quantities  shown  on  the  third  and  fourth  lines. 

No.  1  No.  2  No.  3  No.  4  No.  5  Sludge 

Per  Per  Per  Per         Per       Per  Total 

cent.  cent.  cent.  cent.  cent.     cent. 

1.  Yield  in  per  cent 90  88  85  82  78         40 

2.  Impurities   in  washed 

coal 3  4  6  7  9         15 

3.  Washed  coal  in  tons..      135       158.4    178.5    196.8    748.8    216         1,633.5 

4.  Refuse   in   tons 15         21.6      31.5      43.2    211.2    324  646.5 

Total  in  tons 150       180       210       240       960       540         2,280.0 

plus  lump  coal   720.0 

Total    3,000.0 

TABLE  17 

Table  14  is  used  to  determine  the  size  of  the  screens  if  the  coal 
is  sized  before  washing.  Tables  15  and  16  are  used  as  a  basis 
for  designing  the  jigs.  The  third  line  of  Table  17  shows  the 
daily  quantities  of  the  washed  coal  and  can  be  used  for  deter- 
mining the  size  of  the  loading  bins.  It  is,  however,  advisable 
to  make  the  washed  coal  bins  of  equal  size  and  large  enough  to 
hold  sufficient  coal  to  fill  two  railroad  cars.  Thus  with  five  sizes 
we  will  get  a  washed  coal  bin  of  400  tons'  capacity  divided  into 
five  compartments 'capable  of  holding  80  tons  each. 

If  the  coal  is  sized  after  washing,  the  above  figures  must  be 
used  for  the  screen  dimensions  instead  of  the  figures  in  Table  14. 
Table  17  on  the  4th  line  shows  the  daily  quantities  of  refuse  and 
these  must  be  used  to  determine  the  size  of  refuse  elevator  and 
bins. 

Screens.  The  demands  made  upon  the  screens  used  in  coal 
washeries  are  as  follows: 

(1)  Exact  Sizing.  This  demand  is  required  only  for  final 
screening.  If  the  final  screening  is  not  exact  complaints  from 


114  COAL  WASHING 

consumers  and  the  cutting  of  standard  prices  are  the  results. 
Exact  sizing  will  be  secured  by  a  proper  type  of  screen  and  the 
correct  size  of  the  different  screen  plates.  It  is  important  to 
avoid  a  crowding  of  the  screens,  as  this  renders  exact  sizing 
impossible. 

(2)  Avoiding  Trituration.     A  sliding  motion  of  the  material 
is  most  favorable  to  avoid  breakage  of  coal.     The  exact  sizing  is, 
however,  of  such  importance  that  screens  imparting  to  the  coal 
a  jumping  motion  are  often  used,  since  they  size  accurately.     If 
it  is  considered  that,  at  least  when  the  coal  is  to  be  sized  after 
washing,  the  material  handled,  has  been  subjected  in  the  ele- 
vators, and  the  jigs  to  appreciable  abrasion,  it  can  be  readily 
seen  that  prevention  of  further  breakage  is  not  especially  diffi- 
cult. 

(3)  Capacity.     It  is  only  natural  that  each  piece  of  apparatus 
will  be  used  to  its  fullest  capacity.     Great  care,  however,  must 
be  taken  to  determine  the  limitations  of  the  screens.     The  capac- 
ity of  a  screen  is  fixed  by  the  accuracy  of  sizing,  demanded  by 
the  consumer.     To  force  a  screen  beyond  this  would  be  uneco- 
nomical. 

A  specification  for  a  screening  plant  must  contain  a  clear  defi- 
nition of  what  is  understood  by  "capacity."  An  effort  must  be 
made  to  build  the  screen  for  as  great  a  capacity  as  possible.  In 
most  cases  one  system  of  washing  must  correspond  with  one  type 
of  screens.  The  installation  of  several  types  would  result  in  a 
highly  complicated  arrangement  for  delivering  the  materials 
from  one  piece  of  apparatus  to  another. 

(4)  Assurance  of  Steady  Operation.     The  installation  of  a 
spare  screening  plant  can  not  be  recommended  on  account  of  the 
above  named  complications  and  the  necessity  of  making  the  best 
use  of  the  space  at  disposal.     Consequently  the  screens  must  be 
built  for  uninterrupted  operation,  free  from  breakdowns  and 
repairs. 

(5)  Avoidance   of  Vibration.     The   location   of  the   screens, 
usually  at  the  highest  point  of  the  washery,  and  the  rapid  motion 
required  for  exact  sizing  can  by  an  incorrect  or  unsuitable  con- 
struction cause  a  detrimental  vibration  of  the  whole  washery 
building. 

(6)  Possibility  of  Changing  the  Screen  Plates.     Under  cer- 


CLASSIFYING  OF  FINE  COAL  115 

tain  conditions  it  may  become  advantageous  to  be  able  to  change 
the  size  of  the  coal  by  putting  in  screen  plates  having  different 
perforations.  The  construction  of  the  screens  should  be  such 
that  this  can  be  done  quickly  and  easily. 

(7)  Accessibility.  On  account  of  the  lack  of  spare  screens, 
repairs  must  be  made  quickly,  since  every  repair  of  the  screening 
plant  shuts  down  the  whole  washery. 

Revolving  or  Shaking  Screens.  Practical  experience  has 
demonstrated  that  exact  sizing  can  be  equally  well  obtained  with 
either  revolving  or  shaking  screens.  The  coal  is  handled  more 
gently  on  a  flat  shaking  screen,  since  the  material  here  slides 
over  the  plates,  whereas  in  a  revolving  screen  the  coal  is  carried 
paft  way  up  on  the  inside  of  the  mantle  and  caused  to  roll  back 
again. 

Sufficient  capacity  can  be  obtained  from  both  types.  Shakers 
with  the  same  screening  area  as  revolving  screens  have  a  greater 
efficient  area,  because  with  revolving  screens  only  the  lower  part 
(about  %)  of  the  whole  area  is  used  efficiently.  On  the  other 
hand  the  compact  construction  of  a  revolving  screen  permits 
the  employment  of  considerably  greater  screening  areas.  Re- 
volving screens  give,  on  account  of  the  uniform  motion  and  sim- 
plicity of  the  driving  mechanism,  a  greater  assurance  of  un- 
interrupted operation  than  do  shaking  screens.  Improved  con- 
struction and  careful  design  of  the  modern  shaking  screens  have 
however,  placed  them  on  an  equal  footing  with  revolving  screens 
in  regard  to  continuous  performance. 

Vibration  can  be  almost  entirely  avoided  through  the  use  of 
revolving  screens.  With  shaking  screens  vibration  can  be  re- 
duced but  not  totally  eliminated,  through  the  use  of  balanced 
screens  and  flexible  hanger  rods. 

Accessibility  and  rapid  change  of  screen  plates  are  difficult  to 
obtain  with  revolving  screens  but  offer  no  difficulty  with  shaking 
screens.  Besides  the  above,  we  must  consider  the  amount  of 
power  required,  which  is  somewhat  smaller  for  revolving  than 
for  shaking  screens. 

The  above  discussion  shows  that  no  universal  decision  can  be 
reached.  In  condensing  the  different  considerations  the  follow- 
ing axioms  can  be  obtained : 

Shaking  screens  are  to  be  preferred  if  the  possibility  of  chang- 


116 


COAL  WASHING 


ing  the  screen  plates  and  the  accessibility  in  case  of  repairs  are 
to  be  considered  of  primary  importance.  If  absolute  assurance 
of  continuous  operation,  absence  of  vibration  and  low  power 
consumption  are  more  important,  revolving  screens  are  to  be 
recommended. 

In  Illinois  1  the  sizing  of  raw  coal  prior  to  washing  is  carried 
on  on  12  revolving  screens  and  2  shaking  screens,  and  the  sizing 
of  the  washed  coal  on  35  revolving  and  20  shaking  screens. 

It  has  been  previously  explained  (page  7)  that  the  question  of 
screening  from  fine  to  coarse  or  from  coarse  to  fine  has  no  funda- 
mental importance,  and  that  the  space  at  disposal  and  the  most 


Fig.  56.     Triple-Jacketed  Revolving  Screen 

suitable  arrangement  are  of  greater  importance.  Screening 
from  coarse  to  fine  gives  the  advantage  that  the  material  is  re- 
ceived on  the  screen  plates  having  the  largest  perforations, 
thereby  saving  the  finer  mesh  screens  from  much  wear.  This 
system  of  screening  demands,  with  revolving  screens  a  construc- 
tion of  concentric  plates  or  the  arrangement  of  separate  screens 
for  each  size  of  coal,  because  the  undersize  from  each  screen 
must  be  further  separated  into  additional  different  sizes.  We 
have,  therefore,  the  choice  between  a  compact  but  inaccessible 
apparatus  and  a  series  of  separate  screens  arranged  at  different 
levels.  With  shaking  screens  we  meet  the  same  conditions. 
In  screening  from  fine  to  coarse,  screen  plates  with  different 

i  "Coal  Washing  in  Illinois,"  by  F.  C.  Lincoln.     Bulletin  No.  69,  Engi- 
neering Experiment  Station,  University  of  Illinois. 


CLASSIFYING  OF  FINE  COAL 


117 


perforations  can  be  placed  in  one  mantle  of  a  revolving  screen 
or  in  one  shaker  frame.  This  makes  the  screens  much  simpler 
and  more  accessible.  A  limit  is  given  by  the  required  length  of 
the  screen,  so  that  if  a  good  many  sizes  must  be  made,  a  division 
into  separate  screen  units  is  required  which  destroys  simplicity. 
The  use  of  shaking  or  revolving  screen  units  with  only  one  size 
of  perforation  is  not  to  be  recommended,  since  it  increases  the 
cost  of  the  complete  installation,  which  can  be  avoided.  The 


Fig.  57.     Triple- Jacketed  Revolving  Screen 

sizing  from  coarse  to  fine  in  concentric  revolving  screens  or  in 
multiple  shaking  screens  is  advantageous  if  a  good  many  sizes 
must  be  made  in  the  least  possible  space. 

The  sizing  from  fine  to  coarse  in  revolving  or  shaking  screens 
having  different  perforations  in  one  plane  can  be  of  advantage 
only  if  but  a  few  sizes  are  to  be  made. 

Types  of  Revolving  Screens.  Only  concentric  revolving 
screens  will  be  here  described.  Single  jacketed  revolving  screens 
are  seldom  used  in  coal  washeries. 


118 


COAL  WASHING 


The  shaft  "a"  rests  in  bearings  upon  supporting  beams  "b," 
but  could  with  equal  ease  be  hung  from  above.  This  shaft  car- 
ries three  spiders  "  d  "  having  six  arms  each.  These  spider  arms 
support  the  rings  '  V  which  carry  the  screen  plates.  Coal  en- 
ters the  screen  at  "  f . "  The  coarsest  size  leaves  the  screen  at  IV 
and  the  finest  at  "I."  N  jackets  make  n  +  1  sizes.  The  screen 


Fig.  58.     Shaking  Screen  for  Three  Sizes  of  Coal 

can  be  driven  either  from  the  right  or  left  side  by  means  of 
gears  "  c. "  The  shaft  may  be  omitted  -and  the  screen  carried  on 
rollers.  Screens  of  the  above  type  are  in  extended  use  and 
operate  well. 

Shaking  Screens.     Shaking  screens  are  used  mostly  for  screen- 


Fig.  59.     Cross  Section  of  Shaking  Screen 

ing  from  fine  to  coarse,  since  only  a  few  sizes  can  be  made  on 
one  screen.  Figs.  58,  59  and  60  show  a  shaking  screen  making 
three  sizes.  Such  screens  can  be  used  for  the  re-sizing  of  the 
washed  coal  and  are  usually  installed  on  top  of  the  washed-coal 
bins. 

Fig.  61  shows  such  an  installation.     The  washed  coal  from 


CLASSIFYING  OF  FINE  COAL 


119 


3  in.  to  0  in.  is  sluiced  in  the  launder  "a"  onto  the  upper  screen. 
All  coal  smaller  than  1H  in.  passes  with  the  wash  water  through 


Fig.  60.     Top  View  of  Shaking  Screen 


the  screen  perforations  in  plate 

onto  the  second  screen.     Coal  bigger  than  B4  in.  passes  to  the 


"c"  into  the  spout  "d"  and 


Fig.  61.     Shaking  Screen  Installation  on  Top  of  Washed  Coal  Bins 

screen  plate  "e"  of  the  first  screen.     The  oversize  from  2  in.  to 
3  in.  drops  into  the  spiral  chute  "f "  and  passes  into  bin  No.  1. 


120 


COAL  WASHING 


The  undersize  from  I1/!  in.  to  2  in.  goes  to  bin  No.  2.  On  the 
first  screen  plate  of  the  second  screen,  having  %  in.  perforations, 
the  fine  coal  and  the  wash  water  are  separated  from  the  balance 
of  the  material  and  carried  away  in  the  sluice  "h."  The  under- 
size  of  the  screen  plate  "i"  from  %  to  %  in.  goes  to  bin  No.  4, 
and  the  oversize  from  %  to  1^4  in.  is  deposited  in  bin  No.  3. 

Shaking  screens  with  superimposed  screen  plates  are  used  for 
preliminary  sizing  as  shown  in  Fig.  62.  The  whole  screen  rests 
upon  two  crankshafts  "c"  and  "d"  that  are  connected  by  a 
belt  drive  "e."  The  uniform  motion  of  the  whole  screen  lifts 
the  coal  at  each  stroke  off  from  the  screen  plates  and  throws  it 
forward.  The  subsequent  drop  of  the  coal  onto  the  screen  tends 


Fig.  62.     Shaking  Screen  with   Superimposed  Screen  Plates 

toward  exact  sizing  and  the  forward  motion  of  the  coal  makes  it 
possible  to  place  the  screen  at  a  very  slight  slope. 

Shaking  screens  are  either  supported  on  rollers,  hung  by  rods 
from  above  or  supported  from  above  or  below  by  means  of  planks 
(either  oak  or  ash)  rigidly  attached  to  the  screen  frame.  The 
form  of  shaking  screen  with  the  plank  supports  placed  below  is 
called  the  ' '  Parrish ' '  screen  and  not  only  are  its  supports  of 
wood  fastened  tightly  to  the  frame,  but  its  eccentric  rods  are 
also  of  wood,  firmly  fastened  to  the  screen  body.  This  wooden 
construction  makes  the  Parrish  screen  light,  while  the  rigid  at- 
tachment of  the  planks  and  rods  results  in  a  sharp  upward  jerk 
on  each  stroke  of  the  screen,  which  is  highly  effective. 

The  following  data  on  screens  are  taken  from  "Coal  Washing 
in  Illinois,"  by  F.  C.  Lincoln,  Bulletin  No.  69  of  the  Engineer- 
ing Experiment  Station  of  the  University  of  Illinois. 

The  two  shaking  screens  treating  raw  coal  make  132  strokes 


CLASSIFYING  OF  FINE  COAL  121 

per  minute  and  screen  52  per  cent,  of  %  in.  coal  out  of  S1/^  in. 
screenings  at  the  rate  of  one  ton  per  hour  for  each  0.7  sq.  ft. 
of  screening  surface. 

Twelve  revolving  screens  are  used  for  sizing  the  raw  coal  prior 
to  washing.  Nine  of  them  are  cylindrical  and  three  conical. 
One  is  a  simple  cylindrical  screen  and  one  a  simple  conical 
screen.  Two  are  single  jacketed  cylindrical  screens  making  two 
sizes,  one  is  a  double  jacketed  cylindrical,  four  are  triple  jacketed 
cylindrical,  while  the  remaining  three  are  triple  jacketed  conical 
revolving  sereens.  The  cylindrical  screens  are  placed  at  a  slope 
ranging  from  3  to  7  deg.,  with  an  average  of  5  deg.  The  conical 
screens  have  all  their  axes  in  a  horizontal  position.  The  number 
of  revolutions  per  minute  varies  from  seven  to  30,  giving  periph- 
eral speeds  ranging  from  126  to  471,  with  an  average  of  219  ft. 
per  minute.  The  square  feet  of  screen  surface  per  ton  treated 
per  hour  varies  from  1  to  11.3,  with  an  average  of  4.2.  F.  E. 
Brackett 1  holds  that  this  ratio  should  be  8  when  the  mesh  is 
%  in.  and  16  when  it  is  V±  in.,  indicating  that  the  raw  screens  in 
Illinois  are  not  as  large  as  elsewhere  in  the  United  States. 

For  the  final  screening  of  the  washed  coal  shaking  as  well  as 
revolving  screens  are  used  in  Illinois.  The  shaking  screens  em- 
ployed for  re-sizing  of  the  washed  coal  have  an  average  slope 
of  10V&  deg.,  make  155  strokes — of  4^  in.  length — per  minute, 
and  have  1.35  sq.  ft.  of  screen  surface  per  ton  of  coal  per 
hour. 

The  shaking  screens  employed  for  sizing  coal  not  previously 
sized  while  raw  give  the  following  averages:  Slope,  9  deg., 
strokes,  135  (of  5  in.  length)  per  minute,  square  feet  of  screen- 
ing surface  per  ton  per  hour,  1.7.  We  have  seen  that  0.7  sq.  ft. 
per  ton  of  raw  coal  per  hour  is  the  average  bituminous  practice. 
Wet  coal  requires  more  screening  surface  and  the  Illinois  shak- 
ing screens  conform  to  this  requirement  by  having  twice  that 
area.  The  revolving  screens  used  for  sizing  of  the  washed  coal 
have  an  average  peripheral  speed  of  above  200  and  below  250  ft. 
per  minute.  The  average  square  feet  of  screen  area  per  ton  per 
hour  for  raw  coal  revolving  screens  was  found  to  be  4.2.  Wet 
revolving  screens  should  have  larger  proportional  areas  and  the 
average  of  5.66  and  6.5  sq.  ft.  for  resizing  and  final  sizing  screens, 

iCoal  Age,  Vol.  3   (1913),  page  131. 


122 


COAL  WASHING 


respectively,  appear  at  their  face  to  show  that  this  requirement 
is  carried  out. 

The  following  table  shows  the  percentage  of  the  different  sizes 
produced  in  Illinois  washeries : 


No.  1  No.  2  No.  3  No.  4  No.  5 

Per  Per        Per  Per  Per 

Cent.  Cent.  Cent.  Cent.  Cent. 

Average  for  8  washeries  making  five  sizes  11.85  17.92  18.47  26.72  25.04 

(Central  Field) 

Average  for  4  washeries  making  five  sizes  20.99  21.42  13.25  26.62  11.72 
(Southern  Field) 


The  sizes  to  which  these  proportions  refer  are  indicated  in  the 
following  table: 

Central  Field 

No.  1 

No.  2 

No.  3             No.  4 

No.  5 

5 

3-1% 

1%-1V4 

11/4-%                    %-5/16 

•yio-Vie 

5 

31/2-1%  sq 

%  sq-1  sq  &  1% 

1  sq  &  1%-%  %-i/4 

]/4-0 

5 

31/2—  2i/4 

2V4—  1V4 

lV4-%              %-% 

%—  0 

5 

3-1% 

1%-1V4 

lV4-%             %-5/i6&i/4 

5/16  &  ]/4-0 

5 

3-1% 

1%-lVs 

11^^34             %—  Vi 

•14—  0 

5 

3-1% 

1%-1 

1-%  &  1/2             %  &  1/2-V4 

]/4-0 

| 

3-1% 

1%-lVs 

1%-%             %_i4 

l/4-VlG 

5 

2  &  11/4-1V2 

1V2—  1 

1-%                        %-V4 

V4—  %G 

5 

3-1% 

1%-lVs 

lVs-%             %-% 

%-1/lG 

5 

3-1% 

l%-li,4 

P/4-%                     %-% 

%-0 

5 

3-1% 

1%-1 

1-%                %-% 

1/4-0 

5 

3-2 

2-1% 

l^-n/ic           "/is-^ 

1/4-1/16 

Southern  Field 

5 

3-2 

2-1 

1-%                        %-V4 

V4-0 

5 

3-1% 

l%-!1/4 

1%-%                     %-7/16  &  % 

7lG  &  %-0 

5 

3V2-1% 

1%-1 

1-%                 %-9io&%&* 

L>  sq  •'liG&M&i^  sq-0 

5 

3-2 

2-1 

1-%                        %-V4 

V4-0 

CHAPTER  XIV 
THE  REMOVAL  OF  TRAMP  IRON 

In  the  mining  of  coal  foreign  substances  are  mixed  with  the 
coal  and  carried  with  it  through  the  washery.  It  is  difficult  to 
avoid  this.  Especially  harmful  are  pieces  of  iron.  They  cause 
trouble  and  wrecks  in  the  crushing  plants,  and  work  havoc  with 
the  conveyors  and  feeders.  Furthermore,  on  account  of  their 


MAiSNfT/C  PW.LCY 


OPERATION 


MA6NETJC    PULLC/ 


Fig.  63.     Magnetic  Pulley 

heavy  weight  they  remain  on  the  jig  screens,  where  they  form 
a  heavy  bed  that  prevents  the  required  loosening  up  of  the  ma- 
terials by  weakening  the  water  pulsation.  This  is  detrimental  to 
the  effective  operation  of  the  jigs. 

If  a  mine  sends  out  a  good  deal  of  tramp  iron,  it  is  advisable 
to  install  a  separate  apparatus  to  catch  all  this  foreign  material. 

123 


124 


COAL  WASHING 


On  account  of  the  strong  magnetic  properties  of  iron  a  magnetic 
separator  is  the  logical  selection.  Magnetic  separators  are  sim- 
ple and  offer  no  difficulties.  Any  well-designed  magnetic  sepa- 
rator is  adapted  for  this  purpose,  only  it  must  be  designed  to 
handle  great  quantities.  Magnetic  separators  are  mainly  of 
the  revolving  type  and  are  either  located  in  the  bottom  of  a  chute 
or  in  the  head  pulley  of  a  belt  conveyor.  Revolving  magnets 
have  the  advantage  that  they  deliver  the  attracted  iron  auto- 
matically into  a  separate  chute.  Sometimes  flat  magnets  are 
hung  above  the  chutes,  but  in  this  case  the  attracted  iron  must 
be  removed  by  hand. 


Fig.  64.     Magnetic  Separator 

The  operating  principle  of  these  machines  is  very  simple.  The 
material  to  be  separated  is  fed  upon  a  belt  conveyor  passing  over 
a  magnetized  pulley.  The  non-magnetic  material  falls  by  grav- 
ity from  the  brow  of  the  pulley  vertically  into  a  suitable  recep- 
tacle or  to  a  conveyor  leading  to  final  delivery,  while  the  iron  and 
magnetic  material  are  attracted  and  held  firmly  against  the  belt 
until  it  is  carried  to  the  point  where  the  belt  leaves  the  pulley  on 
the  under  side  and  is  there  discharged  back  of  a  partition  set  a 
few  inches  beneath  the  pulley  in  line  with  its  axis. 

If,  however,  the  coal  is  handled  by  an  elevator,  a  magnetic 
separator  can  easily  be  installed  between  the  elevator  head  and 
the  discharge  chute.  Such  an  apparatus  is  shown  in  Fig.  64. 

The  vital  and  expensive  part  of  such  a  separator  is  the  pulley 


REMOVAL  OF  TRAMP  IRON 


125 


magnet  and  its  present  design  is  the  development  of  years  of 
experience  in  building  this  class  of  machinery.  The  face  of  a 
magnet  is  made  "crowning"  which  keeps  the  conveyor  running 


Fig.  65.     Magnets  Installed  Over  Shaking  Screen 

central  without  guides  or  other  troublesome  devices.  Heavy 
bronze  non-magnetic  end  plates  prevent  iron  and  steel  particles 
from  attaching  themselves  and  clinging  to  the  ends  of  the  pulley.. 
The  magnet  is  built  upon  a  hollow  shaft  which  provides  means 


Fig.  66.     Top  View 

for  conducting  the  wires  connecting  the  energizing  coils  with  the 
contact  parts.  The  contact  parts  are  placed  outside  of  one  of 
the  bearings.  This  permits  of  a  substantial  housing  convenient 


126 


COAL  WA8HINO 


and  easily  opened  for  inspection.  The  separator  is  equipped 
with  driving  pulley,  take-up  boxes,  slate  switch  panel,  pilot  lamp, 
kick-absorbing  switch,  steel  housing  for  contact  parts,  bilge 
boards,  etc. 

If  the  raw  coal  is  sized  before  washing,  the  magnets  can  be 
readily  installed  over  the  screens  as  shown  in  Figs.  65  and  66. 


Fig.  67.     Magnets  Hung  Over  Shaking  Screen 


The  magnets  "b"  are  hung  from  a  shaft  "d"  from  about  4  to 
6  in.  above  the  screen  plates  and  kept  in  place  by  a  rope  "e." 
The  coal  is  forced  by  the  guide  plates  "f"  to  pass  under  the 
magnets.  At  certain  intervals  the  magnets  are  lifted  and  the 
adhering  tramp  iron  removed.  Fig.  67  shows  a  photograph  of 
such  an  installation. 


CHAPTER  XV 
WEIGHING  AND  SAMPLING  APPARATUS 

For  a  proper  control  of  the  process  it  is  advisable  to  install 
continuous  automatic  weighing  apparatus  for  the  raw  coal  and 
for  the  washed  product.  If  belt  conveyors  are  used  to  convey 
the  coal  the  installation  of  such  apparatus  is  simple.  Fig.  68 
shows  the  "Merrick  conveying  weigher "  installed  in  a  belt  con- 


Fig.  68.     Merrick  Conveyor  Weightometer 

veyor,  with  front  sheet  of  casing  removed.  Fig.  69  shows  the 
installation  under  working  conditions.  The  integrator  and 
counter  can  be  seen  through  the  window. 

The  Merrick  conveyor  weightometer  solves  in  an  economical 
manner  the  problem  of  weighing  bulk  material  while  it  is  car- 
ried on  a  belt  conveyor  and  of  making  a  record  of  the  weight  of 
the  passing  load  without  stopping  its  flow. 

Several  of  the  troughing  idlers  which  support  the  conveyor 

127 


128  COAL  WASHING 

belt  are  placed  upon  suspension  angles  which  hang  from  suspen- 
sion rods.  These  rods,  by  knife-edged  pivots,  are  connected  with 
a  system  of  levers,  which,  in  turn  are  connected  with  the  weighing 
beam.  This  beam  is  automatically  brought  to  poise  for  different 
loads  on  the  conveyor  by  a  steel  cylinder  hanging  from  the  beam 
and  floating  in  mercury.  As  the  loads  vary  the  cylinder  floats 
at  different  levels  on  the  mercury,  permitting  the  beam  to  tilt  in 
proportion  to  the  weight. 

A  slender  rod  attached  to  the  end  of  the  weighing  beam  above 
connects  it  with  the  weight-indicating  mechanism.     A  frame  sup- 


Fig.  69.     Merrick  Conveyor  Weightometer 

ports  this  mechanism,  and  is  so  pivoted  that  the  changes  in  the 
position  of  the  weighing  beam  are  communicated  to  it  by  the 
connecting  rod  just  mentioned,  so  as  to  tilt  it  to  a  degree  which 
depends  on  the  deflections  of  the  weighing  beam. 

Mounted  across  the  bars  of  the  tiltable  frame  is  the  shaft  of 
the  weight-indicating  disc.  While  free  to  revolve  between  the 
bars,  the  disc  is  forced  to  tilt  with  the  frame  to  which  it  is  at- 
tached. In  accordance  with  the  weight  on  the  conveyor  this  disc 
is  tilted  from  the  vertical  to  a  varying  degree. 

Into  the  edge  of  the  disc,  all  around  the  periphery,  small 
rollers  are  set,  which  come  in  contact  with  a  narrow  endless  belt 
at  two  diametrically  opposite  points  of  the  disc's  periphery. 
This  belt  is  driven  in  a  horizontal  circuit  by  a  pulley  deriving 


WEIGHING  AXD  SAMPLING  129 

its  power  from  the  conveyor  belt.  When  no  load  is  on  the  con- 
veyor and  the  disc  correspondingly  stands  vertically,  the  narrow 
belt  runs  at  right  angles  to  the  plane  of  the  disc.  As  soon  as  the 
disc  is  tilted  by  the  conveyor  load,  its  position  relative  to  the 
driving  belt,  with  which  it  is  in  contact,  will  change.  Even  a 
slight  tilt  will  cause  the  belt  to  give  to  the  disc  a  slow  rotary 
motion  and  the  greater  the  tilt,  the  nearer  the  plane  of  the  disc 
coincides  with  the  plane  of  the  belt  circuit,  the  greater  will  be 
the  speed  of  the  disc.  A  five-figure  counter  operated  by  the  end 
of  the  disc-shaft  records  the  rotations  of  the  disc.  By  correctly 
choosing  the  speed  reduction  between  the  conveyor  and  the  nar- 
row driving  belt  of  the  weighing  mechanism,  the  counter  will  in- 
dicate weight  in  pounds,  tons,  kilos,  or  any  other  units. 

This  device,  once  installed  and  properly  adjusted,  does  its 
work  faithfully  and  correctly  and  requires  but  little  attention. 

The  installation  of  a  continuous  weighing  apparatus  is  impor- 
tant for  the  proper  control  of  a  coal  washery.  The  raw  coal,  the 
washed  coal  and  even  the  refuse  should  be  weighed.  At  present 
the  mine  weight  is  accepted  for  the  raw  coal.  If  the  coal  is 
taken  directly  from  the  mine  to  the  washery  this  method  might 
be  excused,  but  since  in  most  cases  a  storage  bin  is  placed  be- 
tween the  mine  and  the  washery,  it  is  impossible  to  determine  the 
exact  amount  of  raw  coal  entering  this  plant.  The  weight  of 
the  washed  coal  is  mostly  determined  by  weighing  the  outgoing 
railroad  cars  or  if  the  washed  coal  is  taken  in  larries  to  the  coke 
ovens,  by  the  number  of  larries.  These  are,  of  course,  supposed 
to  be  uniformly  loaded.  This,  however,  does  not  always  give 
the  exact  amount  of  washed  coal  produced,  since  some  of  the  coal 
remains  in  the  bins. 

Refuse  is  hardly  ever  weighed.  The  number  of  refuse  cars 
are  marked  on  a  tally  board  and  an  arbitrary  weight  taken  for 
the  contents  of  each  car.  By  installing  continuous  automatic 
weighing  machines  for  the  raw  coal,  the  washed  coal  and  the 
refuse  a  close  control  can  be  established  over  the  washing  process 
and  guess  work  and  haphazard  methods  eliminated.  The  Mer- 
rick  weightometer  is  guaranteed  to  give  99  per  cent,  accuracy 
and  to  maintain  without  trouble  continuous  24-hr,  service. 

Automatic  Sampling  Apparatus.  Besides  the  utility  of  know- 
ing the  correct  weight  of  all  materials  entering  and  leaving  a 


130 


COAL  WASHING 


washery  it  is  of  equal  if  not  greater  importance  to  collect  a  cor- 
rect average  sample  of  the  products.  At  present  sampling  is 
mostly  done  by  boys,  who  take  samples  of  the  raw  coal,  the 
washed  coal,  the  refuse  and  the  sludge  at  regular  intervals.  In 
ore-dressing  plants  the  sample  boy  has  long  ago  been  replaced 
by  automatic  sampling  machines  and  it  is  high  time  that  similar 
methods  should  be  introduced  into  coal  washeries.  In  the  gen- 
eral lay-out  of  a  coal  washery  provision  ought  to  be  made  to 
permit  the  installation  of  sampling  machines.  For  the  sampling 
of  raw  or  washed  coal  and  the  refuse  a  sampling  machine  similar 
to  the  one  shown  in  Fig.  70  may  be  employed. 


Fig.  70.     Intermittent  Sampling  Machine   (Built  by  Colorado  Iron 
Works  Co.) 

This  machine  is  a  modification  of  the  "Vezin"  sampler  and 
takes  a  comparatively  small  sample  at  long  intervals.  It  can  be 
geared  so  that  a  sample  is  taken  every  15,  20  or  30  min.  during 
the  day.  The  samples  are  collected  in  a  receptacle  which  can  be 
locked  and  sealed.  This  machine  can  be  installed  at  the  head  of 
an  elevator  or  conveyor  and  the  sample  taken  across  and  through 
the  stream  of  coal.  For  the  sampling  of  sludge  and  dirty  water 
a  tailing  sampler  operated  by  a  stream  of  water  can  be  used. 
This  apparatus  is  shown  in  Fig.  71. 

The  tailings  sampler  here  shown  is  placed  at  any  convenient 
point  and  causes  practically  no  loss.  It  is  operated  by  a  stream 
of  water,  the  flow  of  which  regulates  the  frequency  with  which 
a  sample  is  taken.  The  tank  at  the  top  of  the  machine  is  divided 
into  two  compartments,  one  on  each  side  of  the  point  of  suspen- 


WEIGHING  AND  SAMPLING 


131 


sion,  which  serve  for  the  operation  of  the  machine.  The  stream 
of  water  enters  one  compartment  until  the  equilibrium  is  over- 
come, when  it  causes  the  splitter  to  sweep  slowly  across  the  end 
of  the  launder  and  take  an  even  sample  clear  across,  depositing  it 
in  a  suitably  placed  box  or  pan. 

The  side  of  the  tank  which  is  weighted  has  now  become  the 
low  side  and  the  water  escapes  gradually  through  a  small  hole 
while  the  opposite  side  is  filling.  By  varying  the  flow  of  water 


Fig.  71.     Sampling  Machine  for  Tailings 


the  frequency  of  sampling  can  be  varied  within  wide  limits, 
20  gals,  per  hour  sufficing  to  operate  it  when  taking  samples  at 
five-minute  intervals. 

The  cutter  of  a  sampler  must  pass  through  the  stream  of  coal 
in  such  a  way  as  to  take  an  equal  proportion  of  all  parts  of  it. 
The  only  safe  way  of  passing  the  sample  cutter  through  the 
stream  of  coal  is  with  its  edge  in  a  plane  at  right  angles  to  the 
long  axis  of  the  stream,  the  cutter  entering  at  one  portion  and 
passing  with  uniform  motion  entirely  through  and  out  at  the 
opposite  portion.  It  will  be  safer  to  pass  the  cutter  through  the 
stream  from  side  to  side,  discharging  from  a  spout  than  from 
front  to  rear  or  rear  to  front,  for  in  the  latter  cases  unless  the 
cutter  is  introduced  into  the  stream  at  some  distance  below  the 


132  COAL  WASHING 

point  of  discharge  the  reaction  of  the  large  grains  striking  the 
edge  of  the  cutter  will  tend  respectively  to  throw  them  into  the 
cutter  or  away  from  it,  whereas  for  accurate  sampling  there 
should  be  no  marked  tendency  in  this  respect  one  way  or  the 
other.  When  wet  coal  is  being  sampled,  the  interior  of  the  cut- 
ting device  should  be  cleaned  at  proper  intervals  for  the  wet 
coal  tends  to  cling  to  the  sides  in  the  rear  portion  where  the  fine 
coal  falls  in  a  side  to  side  cut,  eventually  leaving  only  a  confined 
space  and  reducing  the  proportion  of  fines.  The  unvarying 
frame  of  mind  of  those  having  charge  of  sampling  should  be  one 
of  suspicion.  Due  care  should  be  exercised  that  all  sampling 
apparatus  is  kept  clean  and  running  freely.  It  is  an  excellent 
plan  to  mount  a  revolution  counter  upon  the  samples,  or  on 
gearing  driving  them  to  record  revolutions  during  one  shift. 

It  is  of  equal  importance  that  provisions  for  the  installation 
of  automatic  samplers  should  be  made  in  the  arrangement  of  the 
machinery.  Automatic  samplers  ought  to  be  part  of  the  installa- 
tion just  as  much  as  jigs,  elevators,  conveyors,  etc. 


CHAPTER  XVI 
PREPARATORY  INVESTIGATIONS 

The  more  or  less  successful  selection  or  construction  of  a  jig 
determines  the  economic  success  of  a  washery.  The  commercial 
results  of  a  washery  are  influenced  by  the  efficient  yield  of  the 
jigging  process.  The  yield  of  a  washery  is  the  proportion  of  the 
washed  coal  to  the  raw  coal,  or  of  the  output  to  the  input.  A 
washery  which  produces  1200  tons  of  washed  coal  from  1500  tons 
of  raw  coal  has  a  yield  of  1200  -f-  1500  X  100  =  80  per  cent. 
This  yield  depends  upon  the  ash  content  of  the  washed  coal. 

It  is  impossible  to  make  an  absolutely  perfect  separation  of 
coal  from  refuse.  Some  refuse  will  be  carried  over  with  the 
washed  product  and  a  certain  percentage  of  good  coal  goes  into 
the  refuse.  This  imperfection  of  jigging  brings  about  a  decrease 
in  the  yield  which  is  in  direct  proportion  to  the  reduction  of  ash. 
The  possible  yield  with  a  predetermined  amount  of  ash  is  influ- 
enced by  the  composition  of  the  raw  coal  and  can  only  be  deter- 
mined in  each  special  case  by  a  thorough  investigation.  If  the 
possible  limits  have  been  determined  consideration  must  be  given 
to  the  selling  price  of  washed  coals  having  different  percentages 
of  ash.  The  yield,  percentage  of  ash  and  the  selling  price  must 
be  considered  together  in  order  to  arrive  at  the  maximum  total 
value  of  the  washed  coal. 

A  typical  example  will  illustrate  this.  Let  us  assume  that  the 
possibility  of  putting  a  coal  on  the  market  commences  at  8  per 
cent,  of  ash.  The  assumed  price  of  coal  with  this  ash  content  is 
taken  at  $3  per  ton,  and  for  each  1  per  cent,  decrease  in  ash  the 
selling  price  advances  25c.  per  ton.  Experiments  gave  the  fol- 
lowing results :  An  8  per  cent,  ash  gave  95  per  cent,  yield ;  a 
6  per  cent,  a  90  per  cent,  yield,  and  a  4  per  cent,  ash  gave  an 
85  per  cent,  yield.  The  price  for  the  different  coals  will  there- 
fore be  as  follows: 

133 


134  COAL  WASHING 

95  X  3.00 

8  per  cent,  ash  coal  =-  -  =  $2.85 

100 

90X3.25 

6  per  cent,  ash  coal  =  -  -  =  $2.925 

100 

85  X  3.50 

4  per  cent,  ash  coal  =  — =  $2.80 

100 

This  shows  that  the  best  returns  would  be  received  by  washing 
the  coal  down  to  6'  per  cent.  ash. 

The  results  secured  from  the  tests  for  the  yield  and  percentage 
of  ash  can  be  used  to  calculate  the  composition  of  the  different 
products  if  the  amount  and  percentage  of  ash  in  the  raw  coal 
are  known.  If  3,000  tons  of  raw  coal  with  10  per  cent,  ash  are 
to  be  washed,  we  get  the  following  results : 


Composition  of  Composition  of 
Washed   Product  Refuse 
Com-  Com- 
bustible,  Ash,  Total  bustible,   Ash,  Total 
Tons       Tons  Tons  Tons     Tons  Tons 

With   8  per  cent,  ash,   95  per  cent,  yield     2,622        228  2,850  78          72  150 

With  6  per  cent,  ash,   90  per  cent,  yield     2,538        162  2,700  162        138  300 

With  4  per  cent,  ash,   85  per  cent,  yield     2,448        102  2,550  252        198  450 

The  figures  given  in  the  table,  even  if  exact  conditions  are  not 
known,  permit  us  to  draw  a  conclusion  in  regard  to  the  compo- 
sition and  nature  of  the  raw  coal.  The  raw  coal  is  comparatively 
clean,  containing  only  10  per  cent.  ash.  The  impurities  are 
mostly  bone  coal.  This  can  be  judged  by  the  large  percentage 
of  good  coal  in  the  refuse,  which  can  only  be  caused  by  a  slight 
difference  in  the  specific  gravity  between  the  coal  and  refuse. 

The  foregoing  example  is  carried  out  for  unsized  coal.  If 
sized  coal  is  to  be  studied,  determinations  for  each  size  must  be 
made  separately.  In  actual  practice  many  other  factors  must  be 
considered  which,  however,  cannot  be  expressed  in  such  a  simple 
way  by  means  of  figures.  The  most  important  of  such  considera- 
tions are: 

(1)   The  possibility  of  using  the  refuse  as  boiler  fuel  or  of  re- 


PREPARATORY  INVESTIGATIONS  135 

covering  some  good  coal  from  it,  by  crushing  and  rewashing. 
The  refuse  shown  in  the  example  given  could  be  easily  used  at 
the  mine  for  boiler  fuel.  (2)  As  the  cost  of  washing  becomes 
greater  with  crushing  and  rewashing,  this  cost  must  be  deducted 
from  the  price  derived  from  the  sale  of  the  recovered  refuse. 
(3)  The  possibility  of  making  through  close  washing  an  especially 
clean  and  valuable  coal  that  will  be  in  demand  even  under  ad- 
verse circumstances,  may  change  the  conditions  mentioned  con- 
siderably. This  advantage  can  not  be  expressed  in  figures  and 
should  be  especially  considered  by  those  mines  that  on  account 
of  the  poor  quality  of  their  raw  coal  can  not  compete  in  the 
market  with  producers  better  situated. 

The  economic  operation  of  a  washery  can  only  be  based  upon 
a  complete  and  intelligently  conducted  washing  test.  The  first 
step  in  such  an  investigation  is  to  make  a  chemical  survey  of  the 
mine.  This  consists  in  making  careful  sections  of  the  coal  bed 
in  different  portions  of  the  mine,  taking  samples  of  the  coal  and 
impurities  in  the  proportion  in  which  they  exist  in  the  vein  (so- 
called  channel  samples).  These  samples  are  taken  down  to  the 
size  to  which  the  coal  will  be  crushed  at  the  washery.  The  sam- 
ples are  then  mixed  in  equal  proportion  by  weight,  all  fine  mate- 
rial which  passes  through  a  20-mesh  screen  is  screened  out  for 
separate  treatment,  and  the  main  sample  is  separated  or  classi- 
fied by  means  of  heavy  solutions  of  varying  specific  gravity. 

The  objects  sought  by  such  procedure  are,  first,  to  obtain  the 
theoretical  ash  or  fixed  ash  in  the  pure  coal,  and  second,  to  so 
classify  the  impurities  as  to  plainly  show  the  quantities,  specific 
gravity  and  ash  content  of  each  class.  For  example,  a  coal 
which  presents  rather  difficult  washing  problems  shows  the  fol- 
lowing results  for  the  so-called  sink-and-float  test: 


Pure  coal  lighter  than 

1.35 
1  35—1  40 

sp.  gr. 
sp.  gr. 
sp.  gr. 
sp.  gr. 
sp.  gr. 
sp.  gr. 
over, 

69.7 
8.3 
3.6 
2.2 
1.0 
2.2 
13.0 

per 
per 
per 
per 
per 
per 
per 

cent, 
cent, 
cent, 
cent, 
cent, 
cent, 
cent. 

with 
with 
with 
with 
with 
with 
with 

7.11 
14.67 
19.10 
24.20 
28.02 
34.65 
73.00 

per 
per 
per 
per 
per 
per 
per 

cent, 
cent, 
cent, 
cent, 
cent, 
cent, 
cent. 

ash 
ash 
ash 
ash 
ash 
ash 
ash 

Impurities 

.    1  40-1  45 

Impurities    

.    1.45-1.50 
1  50-1  55 

Impurities         

.    1  55-1.75 

Impurities    

.    1.75    and 

100.0 

per 

cent. 

with 

18.08 

per 

cent. 

ash 

From  the  foregoing  it  can  be  calculated  that  the  ash  in  the 
washed  coal  would  be  8.71  per  cent,  if  the  separation  is  made  at 


136  COAL  WASHING 

1.45  specific  gravity  and  9.10  per.  cent,  if  made  at  1.55  specific 
gravity,  assuming,  of  course,  perfect  washing.  In  the  farmer 
case  the  washery  loss  would  be  18.48  per  cent,  and  in  the  latter 
15.2  per  cent.,  assuming  that  the  refuse  is  free  from  coal. 

A  chemical  survey  of  a  mine,  if  properly  conducted,  will  give 
accurate  information  on  the  following  points :  The  amount  and 
character  of  impurities  in  the  run-of-mine  coal;  the  amount  of 
fixed  ash  or  that  in  the  pure  coal;  the  amount  of  rejection  which 
it  will  be  necessary  to  make  with  a  coal  washer  to  produce  any 
desired  quality  of  washed  product ;  the  amount  and  character  of 
impurities,  if  any,  that  could  be  drawn  off  as  an  intermediate 
product  and  used  for  boiler  fuel,  together  with  the  heat  value  of 
such  intermediate  product ;  the  composition  of  the  washed  prod- 
uct that  may  be  expected;  the  size  best  adapted  for  the  separa- 
tion of  the  impurities;  the  units  of  machinery  best  adapted  to 
produce  the  desired  results  with  the  least  construction  cost.  In 
other  words,  such  an  investigation  will  show  the  financial  returns 
that  may  be  expected  from  a  washery. 

The  following  table  gives  the  results  of  a  float  and  sink  de- 
termination made  under  actual  operating  conditions  of  a  pan 
jig  washery  treating  Alabama  coal  of  the  "Big  Seam  Bed": 

ANALYSES  OF  RAW  COAL,  WASHED  COAL  AND  REFUSE 

Ash     Sulphur 

.  Raw  Coal  per          per 

cent.       cent. 

General  Sample  17.40      0.67 

Above  %   in 47.5  per  cent 27.13 

Lighter  than  1.35  sp.  gr 64.9  per  cent 0.72 

Lighter  than  1.45  sp.  gr 5.4  per  cent 21.38 

Heavier  than  1.45  sp.  gr 29.7  per  cent. 

100.0  per  cent. 

Through  %  in 52.5  per  cent 11.70 

Lighter  than  1.35  sp.  gr 85.3  per  cent 8.41 

Lighter  than  1.45  sp.  gr 5.7  per  cent 20.80 

Heavier  than  1.45  sp.  gr 9.0  per  cent. 

100.0  per  cent. 

Washed  Coal 

General  Sample  9.30 

Above  %   in 21.2  per  cent 10.16 

Lighter  than  1  35  sp.  gr 90.1  per  cent 9.67 

Lighter  than  1.45  sp.  gr 6.4  per  cent 19.79 

Heavier  than  1.45  sp.  gr 3.5  per  cent 36.24 

100.0  per  cent. 


PREPARATORY  INVESTIGATIONS  137 

Through  %  in 78  8  per  cent 8.90 

Lighter  than  1.35  sp.  gr 93.8  per  cent 7.60 

Lighter  than  1.45  sp.gr 3.6  per  cent 22.71 

Heavier  than  1.45  sp.gr 2.6  per  cent 38.45 

100.0  per  cent. 

Refuse 

General  Sample 63.84 

Above  %   in 65.1  per  cent 71.85 

Through  %  in 34.9  per  cent 60.73 

Lighter  than  1.35  sp.  gr 15.2  per  cent 11.85 

Lighter  than  1.45  sp.  gr 1.0  per  cent 25.65 

Heavier  than  1.45  sp.gr 83.8  per  cent 71.72 

100.0  per  cent. 
TABLE  18 

The  tables  on  pages  138-39  from  T.  J.  Drakeley's  scientific 
study  of  coal  washing  give  interesting  data  of  the  work  done  by 
different  types  of  coal  washeries  in  England. 

The  following  extract  from  a  report  made  by  David  Hancock — 
preliminary  to  the  installation  of  a  washery — will  shed  some  light 
upon  the  mooted  question  of  the  removal  of  sulphur.  It  shows 
clearly  that  the  bottom-bench  coal  could  not  be  improved  by 
washing. 

The  coal  of  the  Nickle  Plate  bed  occurs  in  two  benches  en- 
tirely different  in  their  characteristics.  The  bed  has  the  follow- 
ing average  section: 

18J/2  in.  of  top  coal,  specific  gravity 1.27 

1  in.  of  slate  parting. 

14^  in.  of  bottom  coal,  specific  gravity 1.48 

34  in.  total  thickness  of  bed. 

The  top  coal  was  sampled  separately  in  each  case,  then  a  sam- 
ple was  taken  of  the  bottom  coal,  including  the  parting,  the  sec- 
tions were  carefully  cut  to  uniform  width  and  depth  from  top 
to  bottom.  This  gave  eight  good  samples,  weighing  about  10 
pounds  each.  In  the  laboratory,  the  samples  were  put  through  a 
screen  having  %  in.,  round  perforations  and  thoroughly  mixed. 

An  equal  weight  of  each  sample  was  then  taken,  the  four  sam- 
ples of  top  coal  being  mixed  to  form  an  average  sample  of  the 
top  bench,  and  the  four  samples  of  the  bottom  coal  similarly 
weighed  and  mixed  to  form  an  average  sample  of  the  bottom 
bench.  These  two  samples  were  then  sized  into  three  sizes,  i.  e., 


138 


COAL 


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06    I  i>l 


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Tl-    CO 


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cs 


CM  !>•  .—  I  h-  O     '    O  CO 


co  o  F-H  ^H  o       co 

CS  lO  O  Tt<  '  00         i—  i 


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I  I  I  I 


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.  SCfcC 
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General  < 

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2 

•          ;     ;     ; 

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CS 

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PREPARATORY  INVESTIGATIONS 


139 


II 


C/l 


10  O     I 
10  I 


CO  01  — '     I  O 

1--.   Tf    O       I  t- 


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140 


COAL  WASHING 


7/s  in.  to  %  in.,  %  in.  to  20  mesh,  and  through  20  mesh.  Separa- 
tions at  1.35  sp.  gr.  were  made  on  each  size.  The  following  re- 
sults were  obtained  from  these  separations : 

TOP  BENCH 


Size 

Amount 
Per  cent. 

Separation    at 
1.35   sp.   gr. 

Analysis  of  Float 

Float  ' 
Per  cent. 

Sink 
Per  cent. 

Ash 
Per  cent. 

Sulphur 
Per  cent. 

%  in.-%  in  
%  in.-20  mesh    
Through  20  mesh   .  .  . 
Average    .    .  . 

30 
60 
10 
100 

95.2 
95.6 
86.0 
94.5 

4.8 
4.4 
14.0 
5.5 

2.35 
2.21 
2.75 
2.35 

0.87 
0.88 
0.88 
0.88 

TABLE  19 
BOTTOM  BENCH 

Size 

Amount 
Per  cent. 

Separation  at 
1.35  sp.  gr. 

Analysis 

of  Float 

Float 
Per  cent. 

Sink 
Per  cent. 

Ash 
Per  cent. 

Sulphur 
Per  cent. 

%  in  —  %  in 

34  2 

31.2 
44.6 
62.5 
41.4 

68.8 
55.6 
37.5 

58.6 

8.25 
7.50 

8.85 
7.88 

3.03 
2.85 
3.20 
2.95 

%  in.-20  mesh    
Through  20  mesh    .  . 
Average    

57.4 
8.3 
100.0 

TABLE  20 

The  average  of  the  entire  bed  was  then  calculated,  taking  into 
consideration  the  thickness  and  specific  gravity  and  all  of  the 
sink  in  1.35  sp.  gr.  mixed  in  the  proper  proportions  to  represent 
the  sink  from  the  entire  bed  and  further  classified  as  follows : 

COMBINED  FLOAT,  ENTIRE  VEIN  INCLUDING  PARTING  AND   CLASSIFICATION 

OF  SINK 


Class 

Amount 
Per  cent. 

Ash 
Per  cent. 

Sulphur 
Per  cent. 

Coal  lighter  than  1.35  sp.  gr  

72.3 

4.20 

1.57 

Impurities  1.35—1.40  sp.  gr  . 

6.4 

12.65 

4.70 

Impurities  1.40-1.45  sp.  gr  ;  .  . 
Impurities  1.45—1.50  sp.  gr  , 

4.8 
4.0 

16.40 
18.90 

5.43 
6.67 

Impurities  1.50—1.55  sp.  gr  

2.0 

23.20 

6.85 

Impurities  1.55-1.75  sp.  gr  
Impurities  1.75  and  over  sp.  gr  

4.4 
6.1 

32.78 
65.23 

5.39 
5.14 

TABLE  21 


To  assist  in  forming  an  idea  of  this  proposition,  I  have  calcu- 
lated from  the  foregoing  results  the  following  figures,  showing 


PREPARATORY  INVESTIGATIONS  141 

what  the  results  would  be  if  perfect  separation  was  secured 
(1)  at  1.35  sp.  gr.,  (2)  at  1.45  sp.  gr.,  and  (3)  at  1.55  sp.  gr., 
and  finally  the  average  raw  coal  for  the  entire  bed : 


Amount 

of 

Analysis 

of    Coal 

Coal 

Refuse 

Ash 

Sulphur 

Per  cent. 

Per  cent. 

Per  cent. 

Per  cent. 

Separation 

at- 

1  35 

sp. 

gr.  .  . 

72.3 

27.7 

4.20 

1.57 

Separation 

at, 

1  45 

sp. 

gr.  .  . 

83.5 

16.5 

5.54 

2.04 

Separation 

at 

1.55 

sp. 

gr.  .  .  . 

89.5 

10.5 

6.31 

2.35 

TABLE  22 

Average  Raw  Coal  Analysis.  Ash:  11.07  per  cent.;  Sulphur: 
2.65  per  cent.  It  will  be  noted  that  when  the  coal  is  crushed  to 
%  in.  size,  a  perfect  separation  at  1.55  sp.  gr.,  which  is  better 
than  could  be  expected  of  any  washer,  would  result  in  a  rejec- 
tion of  practically  10  per  cent,  of  the  product,  a  reduction  of 
ash  from  11.07  per  cent,  to  6.31  per  cent.,  and  a  reduction  of 
sulphur  from  2.65  per  cent,  to  2.35  per  cent. 

The  important  point  brought  out  is  that  the  sulphur,  while 
apparently  in  the  form  of  FeS2  is  in  a  finely  pulverized  condi- 
tion and  intimately  mixed  with  the  coal.  The  float  in  1.35  sp.  gr. 
from  the  bottom  bench  carrying  3  per  cent,  of  this  substance 
does  not  show  any  sulphur  visible  to  the  eye. 

When  the  proposition  of  finer  crushing  is  considered,  it  be- 
comes important  to  know  the  composition  of  the  material  finer 
than  20  mesh  and  what  can  be  done  with  it.  Therefore  I  have 
determined  the  composition  of  material  through  20  mesh  in  these 
samples,  as  follows : 

SEPARATION  AT  1.35  SP.  GR.  OF  MATERIAL  THROUGH  20  MESH 

Amount    of  Analysis  of 


Float 
Per  cent. 

Sink 
Per  cent. 

Float 

Sink 

Ash 
Per  cent. 

Sulphur 
Per  cent. 

Ash 
Per  cent. 

Sulphur 
Per  cent. 

Top  bench 

86  0 

14.0 
37.5 

2.75 

8.85 

0.88 
3.20 

33.88 
42.50 

7.65 

5.78 

Bottom  bench   .  . 

.  .      62.5 

TABLE  23 


It  should  be  noted  here  that  when  the  bottom  bench  coal  is 
crushed  to  20  mesh  size  and  separated  by  a  1.35  sp.  gr.  solution, 
the  float  still  contains  3.20  per  cent,  of  sulphur. 


142  COAL  WASHING 

This  shows  conclusively  that  this  coal  could  not  be  brought  to 
a  coking  standard  by  washing.  Similar  coals  are  found  else- 
where in  the  United  States,  especially  in  the  southern  part  of 
Illinois. 

Reinhardt  Thiessen,  research  chemist  of  the  U.  S.  Bureau  of 
Mines,  proved  conclusively  in  a  paper  presented  before  the 
American  Institute  of  Mining  and  Metallurgical  Engineers,  that 
pyrite  besides  occurring  in  the  coal  in  form  of  balls,  lenses, 
nodules,  continuous  layers  and  thin  sheets,  or  flakes,  occurs  also 
as  fine  microscopic  particles,  or  nodules,  disseminated  through 
the  compact  coal.  Finally  there  is  sulphur  in  coal  in  an  amicro- 
scopic  form  (not  visible  with  an  ordinary  microscope),  prob- 
ably combined  with  the  organic  matter  that  exists  in  the  coal. 
The  microscopic  particles  of  pyrite  vary  in  diameter  from  a  few 
microns  to  a  hundred  microns,  the  majority  measuring  from  25 
to  40  microns.  (A  micron  is  a  unit  of  length  equal  to  0.001 
millimeter,  or  about  0.00004  inch.)  Thiessen  further  states  that 
a  certain  amount  of  sulphur  has  been  found  to  be  present  in  coal 
in  an  amicroscopic  form.  Although,  in  certain  samples,  no  sul- 
phur can  be  detected  ordinarily  by  the  microscope,  microchemical 
and  qualitative  chemical  tests  reveal  sulphur.  Recent  observa- 
tions and  analyses  of  a  large  number  of  samples  from  different 
coals  have  shown  that,  in  a  number  of  cases  more  sulphur  is 
found  than  can  be  accounted  for,  if  this  material  were  combined 
only  with  the  minerals  found  in  coal.  This  form  of  this  element 
is  probably  that  recognized  as  organic  sulphur.  Little  or  noth- 
ing is  known  about  it  except  that  sulphur  exists  in  this  form. 
There  are,  however,  a  number  of  observations  on  record  that  lead 
one  to  assume  that  it  is  present  as  such.  The  above  investigation 
of  the  bottom  bench  of  the  Nickle  Plate  bed  tends  to  strengthen 
this  belief  and  the  following  analyses  of  coal  from  the  No.  6 
Illinois  bed  shows  that  either  amicroscopic  pyrite  or  organic  sul- 
phur exists  in  this  coal. 

ANALYSIS  OF  FRANKLIN  COUNTY  (ILLINOIS)   COAL 

Ash     11.48  per  cent. 

Sulphur     2.15  per  cent. 

Volatile  matter   37.85  per  cent. 

Fixed  carbon    50.67  per  cent. 


PREPARATORY  INVESTIGATIONS 


Screening  Tests 


143 


On  screen 
Per  cent. 

Ash 
Per  cent. 

Sulphur 
Per  cent. 

Passing 

1  in  
%  in 

Held 
.  .  Held 
.  Held 

on 
on 
on 

1  in 

%  in 

i/,  in 

.  screen  .  . 
.  screen.  . 
screen 

14.30 
11.48 

91  OQ 

13.53 
12.30 
19  78 

1.58 
1.89 
2  0? 

1/2  in 

•  Held 

on 

14  in 

screen 

18  96 

11  50 

9  f)Q 

Passing 

1,4  in  

.  .Held 

on 

Wr  in 

srreen 

15  44 

9  78 

1  92 

Pa^sin01 

%  in 

.  .Held 

on 

10 

mesh 

2  48 

9  43 

1  88 

Passing 
Passing 
Passing 
Passing 
Passing 
Passing 
Passing 
Passing 
Passin0" 

10    mesh.. 
20    mesh.. 
30    mesh.  . 
40   mesh  .  . 
50   mesh  .  . 
60    mesh  .  . 
80    mesh  .  . 
100   mesh 
200    mesh 

.  .  Held 
.  .  Held 
.  .  Held 
.  .  Held 
..Held 
.  .  Held 
.  .  Held 
i.Held 

on 
on 
on 
on 
on 
on 
on 
on 

20 
30 
40 
50 
60 
80 
100 
200 

mesh.  .  .  . 
mesh.  .  .  . 
mesh  .... 
mesh  .... 
mesh  .... 
mesh  .... 
mesh  .... 
mesh  .... 

6.30 
3.19 
1.56 
1.00 
0.37 
0.22 
0.44 
1.22 
1  85 

11.12 
13.04 
14.63 
12.85 
14.65 
15.28 
15.62 
13.83 
13  05 

1.87 
2.09 
2.19 
2.21 
2.42 
2.59 
2.73 
2.43 
2  25 

The  following  float  and  sink  test  of  coal  from  the  same  bed 
shows  also  that  a  large  portion  of  the  sulphur  can  not  be  sepa- 
rated by  mechanical  means. 


Ash 
Per  cent. 

Sulphur 
Per  cent. 

Raw  coal 

13  03 

3  00 

Heavier  than  1.45  sp.  gr  
1  35  to  1  45  sp    gr 

54.22 

18  68 

8.64 
2  65 

Lighter  than  1.35  sp.  gr  

Pulverized  raw  coal  .... 
Heavier  than  1.45  sp.  gr  

7.11 

12.9 

54.7 

2.13 

2.80 
8.25  

.  .  11.02  per  cent,  weight 

1.35  to  1  45  sp    gr 

18  8 

2  65 

318  per  cent  weight 

Lighter  than  1.35  sp.  gr  

7.1 

2.12..  .. 

.  .  85.00  per  cent,  weight 

The  mechanical  arrangement  of  the  jigs  in  a  washery  depends 
entirely  upon  the  character  of  the  coal.  The  specific  gravities  of 
the  materials  are  to  be  primarily  considered.  The  specific  grav- 
ity of  coal  varies  between  1.28  and  1.4,  and  that  of  the  impuri- 
ties usually  lies  between  1.5  and  3,  or  even  higher  in  case  of 
pyrites.  If  the  specific  gravity  of  the  coal  is  close  to  or  even 
overlaps  in  some  instances  the  specific  gravity  of  the  impurities, 
we  have  a  difficult  problem  on  hand;  but  if  there  exists  a  con- 
siderable difference  between  the  specific  gravities  of  the  two 
products  the  washing  will  become  comparatively  easy. 

In  the  latter  case  simple  pieces  of  apparatus  are  sufficient  and 
one-compartment  jigs  can  be  used.  In  the  first  case,  however, 


144  COAL  WASHING 

separate  jigs  with  two  or  even  three  compartments  must  be 
ployed  and  rewashing  must  be  considered.  Furthermore,  it  must 
be  determined  whether  or  not  the  impurities  are  disseminated 
throughout  the  coal.  In  this  case  the  difference  between  the 
specific  gravities  of  the  materials  is  more  or  less  obliterated  and 
rewashing  is  advisable  if  the  middle  product  cannot  be  used  at 
the  mine.  If  the  difference  between  the  specific  gravities  of  the 
materials  is  small  and  at  the  same  time  the  impurities  are  dis- 
seminated throughout  the  coal,  jigging  in  three-compartment  jigs 
would  be  advisable. 

Many  variations  of  conditions  exist,  and  it  is  difficult  to  pre- 
dict which  type  of  preparation  equipment  should  be  used.  The 
arrangement  of  the  jigs  should  be  such  that  the  flow  of  materials 
can  be  changed  easily. 

As  to  the  actual  performance  of  washers,  it  may  be  said  in  a 
general  way  that  impurities  lighter  than  1.5  specific  gravity  are 
rarely  separated  by  them  to  any  considerable  extent,  and  that  a 
plant  can  be  so  designed  as  to  eliminate  practically  all  impuri- 
ties heavier  than  1.75  specific  gravity  and  make  a  rejection  of  the 
larger  portion  of  material  between  1.5  and  1.75  specific  gravity. 
There  are  few  washers  in  operation,  however,  at  the  present  time 
that  are  doing  so  well.  Also,  if  the  coal  is  not  too  fine  there 
should  not  be  more  than  5  per  cent,  of  coal  in  the  refuse ;  and  this 
amount  will  usually  be  less  than  1  per  cent,  of  the  raw  coal. 

To  show  the  efficiency  of  a  washer,  David  Hancock,  consulting 
engineer,  has  devised  a  chart  upon  which  can  be  shown,  graphi- 
cally, the  composition  of  the  raw  coal,  washed  coal  and  refuse. 
These  are  plotted  to  scale  and  show  at  a  glance  the  comparative 
efficiency  of  different  washers  as  determined  from  actual  tests. 
A  specimen  of  such  a  chart  is  shown  in  Fig.  72.  It  represents 
the  working  of  a  Stewart  type  of  washer. 

It  is  necessary  to  know  first  the  composition  of  the  raw  coal. 
This  is  determined  by  separations  made  for  each  five  points  of 
specific  gravity  upon  an  average  sample.  On  the  left  side  of  the 
diagram  to  any  convenient  scale  are  laid  off  the  percentages 
found.  For  instance,  in  this  case  the  sample  contained  66.8  per 
cent,  of  coal  lighter  than  1.35  specific  gravity ;  therefore,  at  the 
distance  represented  by  this  figure  the  dotted  horizontal  line  is 


PREPARATORY  INVESTIGATIONS 


145 


drawn  and  marked  1.35  specific  gravity.  Also,  the  first  class  of 
impurity  separated  between  1.35  and  1.4  specific  gravity  was 
found  to  be  6.9  per  cent,  of  the  entire  sample,  and  this  distance 
is  laid  off  to  the  same  scale  and  marked  on  the  chart. 

After  laying  off  the  vertical  scale  in  the  same  manner  for  the 
entire  100  per  cent,  of  raw  coal,  the  horizontal  scale  is  then  sub- 
divided according  to  the  per  cent,  of  each  class  of  impurity  found 
in  the  washed  coal  as  compared  with  the  amount  found  in  the 


Fig.  7-9,.    Hancock's  Efficiency  Chart 

raw  coal.  The  balance,  represented  by  the  black  area,  is  the  re- 
jection or  refuse;  both  the  quality  and  amount  are  indicated 
graphically,  the  areas  being  proportional  to  the  weights  of  washed 
coal  and  refuse. 

The  figures  to  the  right  of  the  diagram  show  the  amount  of 
each  class  of  impurity  which  goes  to  waste  and  the  amount  which 
is  retained  in  the  washed  coal.  For  instance,  it  shows  that  0.6 
per  cent,  of  good  coal  is  wasted.  It  shows,  further,  that  practi- 
cally no  separation  of  coal  and  impurities  is  made  below  1.5  spe- 
cific gravity,  and  that  of  the  impurities  heavier  than  1.75  specific 


146  COAL  WASHING 

gravity  14  per  cent,  is  retained  in  the  washed  coal  and  86  per 
cent,  rejected.  It  should  be  noted  in  this  connection  that  heavy 
impurities  when  retained  in  the  washed  coal  are  usually  fine  ma- 
terial that  would  pass  through  a  J/4-in.  screen.  The  rejection  of 
slate  heavier  than  1.75  sg.  is  practically  complete  in  the  coarse 
sizes. 

This  chart  can  be  called  an  ' '  efficiency  chart "  of  a  coal  washer 
and  is  applicable  to  any  type  of  washer  and  any  coal  if  the  figures 
upon  which  it  is  based  are  accurately  ascertained  in  any  given 
case.  In  the  case  of  the  washer  shown  by  the  diagram,  the  ash 
of  the  raw  coal  was  15.94  per  cent,  and  this  was  reduced  by  wash- 
ing to  11.90  per  cent.,  the  coal  being  a  difficult  one  to  wash.  The 
amount  of  refuse  was  10.8  per  cent,  of  the  raw  coal,  and  the 
yield  was  therefore  89.2  per  cent. 

One  other  type  of  graphical  illustration  of  the  washing  process 
has  been  described  by  Pascal  in  the  Colliery  Guardian  (Aug.  10, 
1917).  Messrs.  Thomas  Fraser  and  H.  F.  Yancey  have  made  use 
of  this  graph  for  showing  the  difference  between  washable  and 
nonwashable  coal.1 

The  graphs  here  presented  show  the  analysis  of  a  washable  coal 
before  and  after  washing,  and  of  this  washable  coal  compared 
with  a  raw  coal  which  is  difficult  to  wash.  The  first  graph, 
Fig.  73,  shows  very  clearly  what  class  of  material  is  removed  by 
washing.  While  particles  heavier  than  1.60  specific  gravity  were 
practically  all  removed,  particles  between  1.30  and  1.60  in  spe- 
cific gravity  are  not  appreciably  affected  by  washing.  The 
analyses  given  in  Table  24  show  that  this  material  is  higher  in 
ash  and  sulfur  than  is  desirable  in  the  clean  coal,  but  lower  than 
is  desirable  in  the  refuse.  This  represents  a  class  of  impurities 
difficult  to  remove,  and  is  the  product  that  appears  at  the  wash- 
ery  as  '  *  true  middling. ' '  If  the  specific  gravity  analysis  of  a  raw 
coal  shows  a  large  percentage  of  this  material  it  is  very  difficult 
to  wash  successfully.  This  condition  is  illustrated  in  the  second 
graph,  Fig.  74,  comparing  the  raw  coal  of  Fig.  73  with  a  coal 
much  more  difficult  to  wash.  The  total  percentage  between  1.35 
and  1.6  specific  gravity  on  the  non-washable  coal  is  40  as  com- 
pared with  only  17  for  the  washable  coal. 

i  "Some  Factors  that  Affect  the  Washability  of  a  Coal,"  by  Thomas 
Fraser  and  H.  F.  Yancey. 


PREP  A  RA  TORY  IN  VEST  I GA  TIOXS 


ANALYSES  OF  COALS  REPRESENTED  IN  FIG.  73 


147 


Specific  Gravity 

Per  cent, 
of  total 
sample 

Raw  Coal 

Ash, 
Per  cent. 

Sulphur, 
Per  cent. 

Washed   Coal 

of'total'         Ash'           Sulphur, 
sample       Per  cent'      Per  <«*• 

-1.30      .... 

73.35 

8.74 
4.93 
1.82 

4.64 
11.27 
17.78 
20.32 
24.60 
29.90 
49.53 
84.04 

1.72 
2.14 
2.39 
2.52 
2.62 
2.80 
3.43 
13.63 

85.50 
8.30 
3.70 
0.88 
0.27 
0.54 
0.34 
0.57 

4.77 
11.8 
17.9 
18.5 
23.6 
28.3 
48.8 
80.3 

1.63 
2.06 
2.13 
2.36 
2.55 
2.84 
3.76 
7.07 

1.30  to  1.35  
1  35  to  1.40  

1  40  to  1.45  

1  45  to  1.50 

0.39 
1.12 

1  50  to  1  60      ... 

1  60  to  1.80 

2.13 

1  80-  

7.52 

TABLE  24 
ANALYSES  OF  COALS  REPRESENTED  IN  FIG.  74 


Per  cent. 
Specific   Gravity           of   total 
sample 

Washable  Coal 

Ash,          Sulphur, 
Per  cent.     Per  cent. 

Non-washable 
Per  cent.        A  h 

l^e    ?—  •*• 

Coal 

Sulphur, 
Per  cent. 

.30  to 
.35  to 
.40  to 
.45  to 
.50  to 
.60  to 
80- 

L  30 

.  .      73.35 

4.64 
11.27 
17.78 
20.32 
24.60 
29.90 
49.53 
84.04 

1.72 
2.14 
2.39 
2.52 
2.62 
2.80 
3.43 
13.63 

55.9 
20.5 
11.8 
3.8 
1.8 
2.1 
1.1 
3.0 

10.1 
13.3 
15.4 
19.1 
22.5 
27.6 
42.7 
60.5 

2.91 
3.35 
3.45 
4.39 
6.18 
9.29 
13.30 
34.12 

I  35 

8.74 

.40..  . 
45 

4.93 
1.82 

50 

0.39 

I  60 

1  12 

80 

.    .          2.13 

.  .    .        7.52 

TABLE  25 

The  ideal  coal  for  washing  would  be  represented  by  a  graph 
showing  all  the  material  concentrated  in  the  parts  heavier  than 


Percent  of  Total  Coa 
20          30  40 


Fig.  73.     Graph  Showing  Comparative  Percentages  of  Material  of  Differ- 
ent Densities  in  a  Coal  Before  and  After  Washing  at  0-^4  in.  Size 

1.60  and  lighter  than  1.30.     Results  of  a  washing  test  on  the  coal 
represented  in  Fig.  73  are  given  in  Table  26.     Table  27  gives 


148 


COAL  WASHING 


the  result  of  a  test  on  the  non-washable  coal  of  Fig.  74.  The 
specific  gravity  determinations  on  these  samples  were  made  by 
separating  the  sample  at  1.30  specific  gravity  with  the  Delameter 
sink  and  float  machine  and  treating  the  sink  in  a  succession  of 


246810 


Percent  of  Total  Coal  Sample 
20          30          40          50  GO 


Fig.  74.  Graph  Showing  Comparative  Percentages  of  Material  of  Differ- 
ent Densities  in  a  Washable  Coal  and  a  Non-Washable  Coal,  Both 
Crushed  to  %  in.  Maximum 

heavier  solutions  in  beakers.  The  effect  of  the  conditions  de- 
scribed on  the  results  attainable  by  washing  is  shown  by  washing 
tests  on  some  typical  coals. 

A  Williamson  County,  III.,  coal  is  represented  in  Fig.  73  and 
Table  24  and  is  the  washable  coal  of  Fig.  74.  The  visible  im- 
purities consisted  of  pyrite  bands  and  lenses  as  much  as  1  in.  in 
thickness;  thin  shale  bands  that  hold  together  well  in  water; 
some  fireclay ;  thin  plates  of  pyrite  in  joint  fissures ;  and  an  un- 
usually large  percentage  of  calcite  and  gypsum  in  the  form  of 

WASHING  TEST  ON  A  COAL  FROM  WILLIAMSON  COUNTY,  ILL. 


Sulphur 

Feed, 
per 
cent. 

Ash, 
per 
cent. 

Reduc- 
tion 
in    Ash, 
per 

Pyritic, 
per 
cent. 

Reduc- 
tion  in 
Pyritic, 
per 

Organic, 
per 
cent. 

Total 
Sulphur 
per 
cent. 

Reduc- 
tion  in 
,     total 
Sulphur, 
per 

cent. 

cent. 

cent. 

Raw  coal    .  .  . 

100.0 

14.2 

1.94 

0.76 

2.70 

Washed  coal  .  . 

85.0 

7.2 

49 

1.09 

44 

0.76 

1.85 

32 

Middlings   .  .  . 

6.6 

19.8 

1.80 

0.76 

2.56 

Washed      coal 

and  middling 

combined    .  . 

91.6 

8.1 

43 

1.14 

41 

0.76 

1.90 

30 

Refuse   

7.3 

72.12 

10.75 

Loss   

1.1 

TABLE  26 


PREPARATORY  INVESTIGATIONS 


149 


thin  sheets.  This  coal  was  washed  at  0  to  14  in.  size  on  a  table. 
The  results  are  given  in  Table  26. 

A  coal  from  White  County,  Tenn.,  is  represented  in  the  graph 
of  Fig.  74  as  non-washable.  A  visual  examination  showed  it  to 
contain  little  clean  shale  or  slate  coarse  enough  to  be  liberated 
by  crushing  to  the  size  at  which  coal  is  ordinarily  jigged.  Visible 
pyrite  was  present  both  as  thin  plates  and  as  coarser  bands  or 
lenses.  The  difficulties  in  washing  this  coal  were,  as  indicated 
by  the  graph,  due  to  an  exceptionally  high  percentage  of  mate- 
rial of  intermediate  density,  indicating  that  the  impurities  are 
so  fine  that  even  when  crushed  to  %  in.  size  they  are  not  liberated, 
and  the  exceptionally  high  ash  and  sulphur  content  of  the  lightest 
coal.  The  ash  content  of  the  part  of  this  coal  that  was  lighter 
than  1.30  in  specific  gravity  was  10.10  while  the  corresponding 
increment  of  the  washable  Illinois  coal  analyzed  4.64  per  cent. 
ash. 

The  Tennessee  coal  was  crushed  to  %  in.  maximum  size  and 
treated  on  a  washing  table.  Although  a  good  reduction  in  sul- 
phur in  the  clean  coal  was  secured,  it  was  made  possible  only  by 
taking  a  very  large  middling  product  and  a  large  refuse  low  in 
ash  and  sulphur.  For  these  reasons  the  washing  of  this  coal 
would  not  be  profitable. 

WASIIIXG  TEST  ox  A  COAL  FROM  WHITE  COUNTY,  TENN. 


Sulphur 

Reduc-  Reduc- 

Feed,          Ash,         tion  Pyritic,  tion   in    Organic,         Total 

per             per     in    Ash,  per        Pyritic,       per               per      g  Y:  f- 

cent.         cent.         per  cent.         per          cent.            cent, 

cent.  cent. 


Reduc- 
tion  in 
total 


per 
cent. 


Raw  coal    .  .  . 

100.0 

15.15 

3.60 

1.17 

4.87 

Washed    coal. 

54.6 

11.30       25 

1.85 

51.5        1.17 

3.02 

38 

Middlings  "... 

32.0 

17.90 

4.04 

1.17 

5.22 

Washed      coal 

and  middling 

combined    .  . 

86.6 

13.80         9 

2.65 

29.00      1.17 

3.82 

21.5 

Refuse   

8.2 

36.39 

17.74 

Loss    

5.2 

TABLE  27 


To  sum  up  the  conditions  that  characterize  an  easily  washed 
coal,  the  excess  undesirable  sulphur  and  ash  should  be  present 
in  form  of  shale  or  pyrite  particles  large  enough  to  be  detachable 


150  COAL  WASHING 

from  the  coal,  without  crushing  finer  than  ^4  in.  in  size.  The 
coal,  when  crushed  to  the  proper  size  for  washing,  should  be 
separable  by  a  sink-and-float  test  into  an  increment  heavier  than 
1.6  specific  gravity  and  an  increment  lower  than  1.30  in  specific 
gravity  and  low  in  ash  and  sulphur  content  with  only  a  small 
percentage  of  intermediate  density  between  these  increments. 
The  impurities  that  make  a  coal  difficult  to  wash  are  thin  bands  of 
friable  shale ;  bony  coal ;  carbonaceous  shale ;  thin  filmlike  flakes 
of  pyrite,  calcite,  or  gypsum  in  joint  fissures;  finely  divided  im- 
purities intimately  mixed  with  the  coal,  and  organic  sulphur. 

The  chief  value,  in  coal-washing  investigations,  of  the  determi- 
nation of  organic  sulphur  by  extraction  of  the  sulphate  and  the 
pyritic  sulphur,  lies  in  finding  a  value  below  which  there  can  be 
no  reduction  of  sulphur  content  by  mechanical  processes.  For 
example,  if  the  coal  from  a  given  mine  contains  3  per  cent,  of 
.total,  and  1  per  cent,  of  organic  sulphur,  it  would  of  course  be 
impossible  to  expect  a  washed  product  carrying  less  than  1  per 
cent,  of  sulphur.  Although  this  is  a  self-evident  fact,  it  is  of 
such  importance  in  determining  the  washability  of  a  coal  that 
attention  is  directed  to  it.  It  would  be  inadvisable  to  give  here  a 
definite  figure  for  the  reduction  in  pyritic  sulphur  that  can  be 
expected  with  the  best  modern  coal  washing  machinery.  The 
data  given  indicate  that,  in  some  coals,  one-half  of  the  pyritic 
sulphur  may  be  removed,  but  the  percentage  reduction  would 
vary  markedly  with  different  coals,  depending  on  the  physical 
form  in  which  the  pyritic  sulphur  occurs.  In  any  case,  the  mini- 
mum sulphur  content  that  may  be  obtained  in  the  clean  coal  is 
well  above  the  organic  sulphur  content  because  some  pyrite  occurs 
in  a  very  finely  divided  state  intimately  mixed  with  the  coal. 
For  practical  purposes  in  coal  washing  this,  in  addition  to  the 
organic  sulphur,  may  be  considered  as  fixed  sulphur. 

In  order  to  deduce  a  satisfactory  method  of  calculating  the 
efficiency  of  the  washing  process,  attention  must  be  directed  to 
the  actual  practice.  The  object  of  washing  the  raw  coal  is  to 
concentrate  to  the  utmost  the  valuable  ingredient  (that  is,  the 
float  particles),  so  that  the  washed  coal  shall  be  a  high-quality 
fuel.  It  is,  therefore,  possible  to  determine  for  a  washing  plant 
the  efficiency  with  which  the  quality  of  the  material  is  improved. 
This  is  termed  the  " qualitative  efficiency"  of  the  process, 


PREPARATORY  INVESTIGATIONS  151 

Obviously,  where  the  washed  coal  was  perfectly  clean — that  is, 
composed  solely  of  float  particles — the  qualitative  efficiency  was 
100  per  cent.  It  is,  however,  highly  probable  that  the  plant, 
in  delivering  a  small  quantity  of  pure  coal,  was  rejecting  as 
refuse  quite  a  large  proportion  of  the  raw  coal.  Evidently  a 
considerable  loss  of  float  particles  in  the  refuse  was  taking  place. 
Therefore,  a  second  conception  is  reached  in  that,  for  perfect 
washing,  the  plant  must  recover  quantitatively  all  of  the  float 
particles  so  that  none  escape  as  refuse.  The  effectiveness  with 
which  the  float  particles  are  recovered  is  termed  the  "quanti- 
tative efficiency"  of  the  plant. 

Thomas  James  Drakeley  has  given  in  his  paper  entitled  "Coal 
Washing,  a  Scientific  Study,"  a  method  of  combining  the  quali- 
tative and  quantitative  efficiencies,  so  as  to  obtain  a  result  that 
will  represent  the  general  efficiency  of  a  coal  washery.  A  simple 
case  will  make  this  method  clear.  Supposing  that  the  washer 
receives  100  Ibs.  of  raw  coal  containing  80  per  cent,  float  and 
20  per  cent,  sink  particles,  and  delivers  the  products  given  in 
the  following  table : 


Description 

Raw  Coal 
per  cent. 

Washed  Coal 
per  cent. 

Refuse 
per  cent. 

Float    

80 

90  (76  5  lb.) 

23  33  (   3.5  lb.) 

Sink 

20 

10  (    8  5  lb  ) 

76  67  (  11  5  lb  ) 

Output  percentage. 

100 

85.0 

15.0 

TABLE  28 

Qualitative  Efficiency.  The  concentration  of  the  float  parti- 
cles in  the  raw  coal  is  80  per  cent.,  and  in  the  washed  coal  90 
per  cent.  Hence  the  concentration  is  raised  by  10  per  cent,  out 
of  a  possible  20  per  cent.  Therefore,  the  qualitative  efficiency  is 
10 

-  x  100  =  50  per  cent. 
20 

Quantitative  Efficiency.  In  dealing  with  the  raw  coal,  the 
plant  rejects  15  per  cent,  of  the  weight  as  refuse — of  which  23.33 
per  cent,  is  composed  of  valuable  float  particles.  This  means  that 

15  X23.33 

float  particles  amounting  to  ,  or  3.5  per  cent,  of  the 

100 


152  COAL  WASHING 

total  output,  are  lost.'    But  of  the  raw  coal  the  valuable  float 
particles  only  amount  to  80  per  cent.     Therefore,  the  plant  re- 
covers 76.5  parts  from  a  possible  80  parts.     Hence  the  quantita- 
76.5 

tive  efficiency  is X  100  =  95.63  per  cent. 

80 

General  Efficiency  of  the  Washing  Process.  The  process  re- 
covers 95.63  per  cent,  of  the  float  particles  with  the  quality  im- 
proved by  50  per  cent.  Therefore,  the  general  efficiency  is 
95.63  X  50 

_  =  47.82  per  cent. 
100 

The  foregoing  cases  are  simple  and  in  the  following  some  of 
the  more  complicated  conditions  met  in  actual  practice  will  be 
given. 

QUALITY  AND  OUTPUT  OF  THE  RAW  COAL  AND  THE  WASHED  PRODUCTS 
FROM  A  WASHERY 

Washed  Coal 

Raw  Coal        Slack  Peas          Beans  Nuts         Refuse 

per  cent,      per  cent.      per  cent,    per  cent,     per  cent,    per  cent. 


Float     

....        84.5 

89.5 

92 

94 

95 

12.5 

Sink        

15.5 

10.5 

8 

6 

5 

87.5 

Output 

100  0 

250 

23 

22 

20 

10.0 

TABLE  29 


One  hundred  pounds  of  raw  coal  entering  the  washery  will 
produce  the  quantities  of  washed  coal  shown  in  the  following 
table : 

CALCULATION  OF  THE  PERCENTAGE  COMPOSITION  OF  THE  TOTAL  WASHED  COAL 


Weight  in  pounds                     Class 

Weight  of 
Float    particles               Sink 
in  pounds 

particles 

25                           Slack 
23                           Peas 
22                           Beans 
20                           Nuts 

Totals      90 
Percentage  composition  of  total  washed 

22.38 
21.16 
20.68 
19.00 

2.62 
1.84 
1.32 
1.00 

6.78 
7.53  per  ct. 

83.22 
coal  92.47  per  cent. 

TABLE  30 


The  concentration  of  the  float  particles  in  the  raw  coal  is  84.5 
per  cent,  and  in  the  washed  coal  92.47  per  cent.     Hence,  the 


PREPARATORY  INVESTIGATIONS  153 

concentration  has  been  raised  by  7.97  per  cent,  out  of  a  pos- 
sible 15.5  per  cent.  Therefore  the  qualitative  efficiency  is 
7.97 

-X  100  =  51.42  per  cent. 
15.5 

The  refuse,  of  which  12.5  per  cent,  is  float  material,  amounts 
to  10  per  cent,  of  the  raw  coal.  Hence,  from  100  Ibs.  of  raw 
coal  1.25  Ibs.  of  float  material  is  lost  in  the  refuse.  Therefore, 
the  process  recovers  (84.5  —  1.25)  or  83.25  Ibs.  of  the  84.5  Ibs. 
entering  the  plant.  Hence,  the  quantitative  efficiency  is 
83.25 

_  x  100  =  98.52  per  cent.     Therefore,  the  general  efficiency 
84.5 

51.42  X  98.52 

is : —  =  50.66  per  cent. 

100 

The  qualitative  efficiency  of  the  different  washing  processes 
examined  by  Thomas  J.  Drakeley  varied  from  about  25  to  75  per 
cent.,  and  averaged  58.20  per  cent.  Mr.  Drakeley,  however, 
states  that  he  considers  that,  where  the  qualitative  efficiency  fell 
below  65  per  cent.,  it  might  have  been  raised  to  this  value  by 
making  slight  alterations  in  the  working  of  the  plant.  Perhaps 
60  per  cent,  might  be  deemed  a  good  average  working  for  trough 
washers. 

The  quantitative  efficiencies  averaged  97.89  per  cent.,  and 
varied  from  about  91  to  99.8  per  cent.  The  average  value  of 
97.89  per  cent,  means  that  2.11  per  cent,  of  the  coal  is  being  lost. 
This  value  compares  well  with  the  loss  sustained  in  ore  dressing. 

General  Efficiency  of  the  Washing1  Process.  From  observa- 
tions it  would  appear  that  a  general  efficiency  of  about  65  per 
cent,  should  be  considered  average  coal-washing  practice;  while 
75  per  cent,  is  certainly  excellent  working.  The  average  value 
for  15  washers  was  56.93  per  cent.  It  will  be  seen  that  the  values 
vary  from  25.16  to  72.38  per  cent.  This  enormous  variation  indi- 
cates that  it  is  a  matter  of  immediate  importance  to  place  the 
practice  of  coal  washing  under  scientific  control. 

It  is  obvious  from  the  foregoing  examples  that  in  order  to  cal- 
culate the  general  efficiency  of  the  washing  process  the  output 
must  be  known.  In  some  instances  it  is  not  possible  to  get  the 


154  COAL  WASHING 

FORMULAE  FOB  FIGURING  THE  EFFICIENCIES  OF  A  COAL  WASHERY 

b-a 

Qualitative  efficiency:  X  = X  100  per  cent. 

100-a 

c.r 

&• 

100 

Quantitative  efficiency:  Y  =  — X  100  per  cent. 

a 

XY 

General  efficiency  of  the  washing  process:  per  cent. 

100 
Where  a  is  percentage  of  float  particles  in  the  raw  coal 

b  is  percentage  of  float  particles  in  the  washed  coal 
c  is  percentage  of  float  particles  in  the  refuse 
r  is  percentage  of  the  refuse. 

correct  weight,  especially  of  the  refuse.  J.  R.  Campbell,  chief 
chemist  of  the  H.  C.  Frick  Co.,  has  developed  a  mathematical 
formula,  whereby  the  percentage  of  the  refuse  and  the  washed 
coal  can  be  easily  ascertained  from  the  analyses  alone.  Mr. 
Campbell  calls  the  mathematical  process  an  alligation  alternate. 
The  following  example  will  explain  this  method :  For  instance, 
given  the  analysis  of  the  raw  coal,  washed  coal  and  refuse,  it  is 
comparatively  easy  to  calculate  the  percentage  of  the  refuse, 
thus: 


Description 

Raw  Coal 
per  cent. 

Washed   Coal 
per  cent. 

Refuse 
per  cent. 

- 

Ash    

14.48 

6.98 

55.42 

Sulphur     

3.53 

2.41 

10.40 

Total  impurities            .... 

18.01 

9.39 

65.82 

TABLE  31 

47.81 

9.39       8.62     47.81 X  100  =  84.7  per  cent,  washed  coal 

56.43 

18.01 

8.62    8.62 

X  100  =  15.3  per  cent,  refuse 

65.82     47.81     56.4356.43 

In  a  similar  manner  the  percentage  of  refuse  can  be  calculated 
from  either  the  ash  or  the  sulphur  determination  alone  when 
accurate^  determined  and  the  results  should  agree  within  rea- 
sonable limits.  In  general,  the  mathematical  deductions  are 
more  accurate  than  the  actual  weights  under  ordinary  conditions 


PREPARATORY  INVESTIGATIONS  155 

of  obtaining  the  latter.     The  underlying  principle  of  "alligation 
alternate"  can  be  readily  formulated  as  follows: 

Let  C  be  percentage  of  ash  in  the  raw  coal,  W  that  in  the  washed  coal, 
and  R  be  the  ash  in  the  refuse;   then 

R  — C 

W        C  — W        R  — C  XI 00  =  per  cent,  washed  coal 

R  — W 


C  — W  C  — W 

R         R  —  C X  100  =  per  cent,  of  refuse 

R  — W  R  — W 

R  — W 

Or,  inversely, =  the  ratio  of  elimination,  and  dividing 

C  — W 

100  by  this  figure  gives  the  percentage  of  refuse,  and  100  per 
cent. — the  per  cent,  of  refuse  equals  the  per  cent,  of  washed  coal. 
In  a  similar  manner, 

(C  — W)R 

X  100  =  per  cent,  of  elimination  of  ash  or  sulphur.     Thus,  in 

(R  — W)C 

the  example  cited  before, 
(14.48  — 6.98)  X  55.42 

X  100  =  59.3  per  cent,  elimination  of  ash 

(55.42  — 6.98)  X  14.48 
(   3.53  —  2.41  )X  10.40 

X  100  =  41.3  per  cent,  elimination  of  sulphur. 

(10.40  — 2.41)  X    3.53 

The  percentage  of  reduction  of  the  impurities  from  raw  to 
washed  coal  has  been,  and  still  is,  frequently  cited  as  a  guide  in 
comparing  different  washer  efficiencies.  It  is  certainly  a  very 
unreliable  guide  unless  the  washers  which  are  compared  are 
working  upon  the  same  coal.  It  will  be  evident  to  anyone  upon 
a  little  reflection  that  the  percentage  of  ash  or  sulphur  reduction 
will  depend  more  upon  the  nature  and  amount  of  the  impurities 
in  the  coal  than  upon  the  different  types  of  washers.  I  have  be- 
fore me  figures  showing  an  ash  reduction  from  21.50  per  cent, 
in  the  raw  coal  to  4.50  per  cent,  in  the  washed  coal,  the  work 
being  done  by  a  washer  little,  if  any,  better  than  the  machine 
which  made  a  reduction  from  15.94  per  cent,  to  11.90  per  cent., 
giving  a  reduction  of  79  per  cent,  as  against  25  per  cent.  The 
explanation  is  to  be  found  in  the  difference  in  the  amount  and 


156  COAL  WASHING 

character  of  the  impurities  in  the  coal  from  the  two  mines  and 
not  in  any  essential  difference  in  the  washers. 

Another  result  might  be  cited  where  a  washer  identical  with 
the  one  which  produced  a  reduction  of  ash  from  15.94  per  cent, 
to  11.90  per  cent,  showed  the  following  results:  Ash  in  raw  coal, 
21.4  per  cent. ;  ash  in  washed  coal,  3.08  per  cent.,  or  a  reduction 
of  85.6  per  cent.  It  is  of  course  absurd  to  suppose  that  any  such 
difference  could  have  existed  between  washers  of  identical  con- 
struction, but  the  difference  is  simply  due  to  the  fact  that  the 
two  coals  contain  impurities  quite  different  in  character  and 
amount. 

The  usual  guarantees  given  for  the  performance  of  a  washery 
include  the  qualitative  and  quantitative  efficiency,  by  specifying 
a  maximum  amount  of  "sink"  in  the  washed  coal  and  a  minimum 
amount  of  "float"  in  the  .refuse.  The  amount  of  float  in  the 
refuse  is  either  expressed  as  a  percentage  of  the  refuse  or  better 
still  as  a  percentage  of  the  total  raw  coal  treated.  The  material 
lighter  than  1.35  sp.  gr.  in  the  refuse,  usually  called  good  coal, 
should  never  exceed  1  per  cent,  of  the  raw  coal  and  in  modern 
washeries  working  under  scientific  control  is  brought  as  low  as 
34  per  cent,  of  the  raw  coal. 

This  means,  that  in  a  washery  treating  2,000  tons  of  raw  coal 
per  day  of  10  hr.,  15  tons  of  good  coal  are  lost  each  day  with  the 
refuse. 

The  above  holds  good  only  for  easily  washed  coal,  containing 
little  if  any  bone.  The  line  of  demarcation  between  good  coal 
and  refuse  should  not  be  too  sharply  drawn.  A  range  of  from 
15  to  20  points  in  the  specific  gravity  ought  to  be  interposed  be- 
tween good  coal  and  refuse,  so  that  a  guarantee  should  read  as 
follows:  The  washed^  coal  shall  not  contain  more  than  .  .  . 
per  cent  of  material  heavier  than  1.50  sp.  gr.  and  the  refuse  shall 
not  contain  more  than  ...  per  cent,  of  the  total  raw  coal  of 
any  material  lighter  than  1.35  sp.  gr. 


CHAPTER  XVII 
DIFFERENT  METHODS  OF  WASHING  COAL 

Besides  the  character  of  the  raw  coal,  which  represents  the 
base  upon  which  the  washing  of  the  coal  must  be  established, 
the  type  of  the  washing  methods  to  be  chosen  must  be  considered. 
The  rules  for  the  proper  selection  of  these  methods  have  been 
given  previously.  They  are  in  close  relation  with  the  mechan- 
ical equipment  of  the  jigs.  According  to  the  washing  processes 
and  the  construction  of  the  jigs,  several  main  types  of  plant  can 
be  established.  These  are  shown,  in  the  accompanying  flow 
sheets. 

In  regard  to  the  flow  sheets  shown,  the  following  remarks  may 
be  made:  The  system  of  rewashing  according  to  Type  IV  can 
in  some  instances  also  be  used  when  the  character  of  the  raw 
coal  itself  does  not  require  such  rewashing.  But  when  it  is 
desirable  to  be  independent  of  the  human  factor  and  tjie  con- 
tinuous care  of  the  operator,  especially  when  the  washed  coal 
must  be  exceedingly  clean  and  is  sold  under  strict  specifications, 
the  coal  is  washed  .very  closely  in  the  primary  jigs  arid  the  result- 
ing unavoidable  loss  of  good  coal  in  the  refuse  is  recovered  in 
the  rewash  jigs.  ,  .  ,'v~ 

In  Type  VI  the  rewash  jigs  for  the  middle  products  of  the 
primary  jigs  could  be  located  ahead  of  the  recmshing  plant. 
This  transposition  should  also  be  considered  in  Type  V,  and  its 
advisability  will  depend  much  upon  the  character  of  the  middle 
product.  If  this  contains  a  considerable  amount  of  slate,  pick- 
ing tables  can  be  used  for  the  purpose  of  removing  the  heavy, 
pure  slate,  which  when  crushed  would  interfere  with  the  proper 
operation  of  the  rewash  jigs.  The  foregoing  types  can  be 
changed  to  suit  local  conditions.  Only  the  most  typical  cases 
have  been  selected 


157 


158 


COAL  WASHING 


i 
I 

Raw  Coal  from  Screen* 
Jig*  for  3  in.  Unsized  Coal 

Mm 

Washed  Coal 

Type  I 


Raw  Coal  fromScrMn* 

Jig.  for  3  in.\Jn.ized  Coal 

Rsfu*. 

Washed  Coal 
Sizing  Screen* 

"  V 
X  in.  to  3  in. 

i.                                               I 

X  in.  to  Du«t 
Rewash   Jig* 

lefuse                                           Wa*h*d  Fin*  Coal 

Type  II 


3-in.   Raw  Coal 
Preparatory  Sizing 

X  in.  tt  3  in 
Coarse  Coal  Jig* 

X  »n.  t*   Durt 
Fine  Coal  Jig* 

Refute                    Washed  Coarse  Coal                      Refute 

Washed  Fine  Coal 

Type  III 


3  in.   Raw  Coal 
Preparatory    Sizing 


Type  IV 


Preparatory  Sizing 


X  in.  to  3  in. 


Product       Washed  Coal       Retu**       Middle 


R«ru«hing 


>roduct      Washed  Coal 


Type  v 


DIFFERENT  METHODS  OF  WASHING  COAL 


159 


Preparatory    Sizing 


Fine  Coal  Jiga 


RewuhJiga 


Re/UM  Middle  Product 

Recru.hing 


Rewaah   Jin 


Washed    Fine  Coal 


Type  VI 


Raw  Coal  Cru.hed  to  %  in. 
Two   Compartment   Jiga 


Firat  Compartment 


Clear.  Refua*          Hutch  Work 


Middle  Product 

Second  Compartment 

Hutch  Work       Washed  Coal 


Concentrating  Table* 


Refute 


Type  VII 


Raw  Coal  Crushed  to  X  in. 

Preparatory  Sizing 

1 

X  in.  to  X  in. 

—  v^-  — 

Coarae  Coal  Jiga 

1 
X  in.  to  X  in. 

Fine  Coal  Jiga 
Refuae     '      Waahed  Coal 

X  In.  to  20  mean 
Concentrating  Table* 
Refuae          Waahed  Coal 

Refuae           Wa»hed  Coal 

Type  VIII 


CHAPTER  XVIII 
THE  FEEDING  OF  THE  JIGS 

The  raw  coal  is  fed  fo'the  jigs  direct  from  the  raw  coal  eleva- 
tor if  unsized  coal  is  to  be  washed ;  and  if  the  coal  is  sized  be- 
fore washing,  from  the  discharge  chutes  of  the  screens  either  by 
means  of  gravity  chutes  or  m  sluiceways  with  water.  This 
method,  however,  has  been  largely  abandoned  and  modern  wash- 
ers have  in  the  rear  and  above  the  jigs  small  equalizing  bins  to 
further  secure  an  even  and  uninterrupted  supply  of  coal  to  the 
jigs.  It  is  also  advisable  to  feed  the  coal  to  the  jigs  by  means 
of  mechanically  operated  feeders.  In  most  installations,  the  flow 
of  coal  into  the  jigs  is  merely  regulated  by  a  slide  gate.  This, 
however,  does  not  give  an  even  feed,  especially  with  coarse  coal. 

The  simplest  and  best  type  of  feed  is  a  slowly  revolving  drum 
with  a  slightly  corrugated  surface.  The  speed  of  the  drum 
should  be  adjustable.  This  is  accomplished  most  easily  by  means 
of  a  ratchet  wheel  and  pawl,  actuated  from  the  jig  eccentric 
shaft.  Provision  should  be  made  whereby  the  pawl  may  be 
caused  to  cover  a  greater  or  less  number  of  teeth  on  the  ratchet 
wheel  so  that  the  drum  will  revolve  faster  or  slower  as  desired. 
Shaking  or  oscillating  apron  feeders  are  also  in  use,  "but  these 
are  more  complieatedy-take  up-more  room  and  cannot  be  adjusted 
with  such  nicety  as  the  revolving  d;rum  feeders. 

The  coal  from  the-feeders  should  flaw^into.  the  jigs  i^i  such  a 
way  that  it  will.be  discharged  below  the  surface  of  the  water,  so 
that  all  the* coal  is  totally  submerged.  For  fine  and  dry  coal 
it  is  advisable  t a  spray  the  coal  before  it  enters  the  jigs,  to  pre- 
vent the  formation  of  dry  lumps. 


160 


CHAPTER  XIX 
TYPES  OF  JIGS 

It  has  been  previously  remarked  that  the  construction  of  the 
jigs  depends  upon  the  character  of  the  material  to  be  washed. 
The  jigs  can  be  divided  into  three  main  types — coarse  coal  jigs, 
fine  coal  jigs,  and  jigs  for  unsized  coal.  According  to  the  flow 
sheets,  we  find  also  jigs  making  only  two  products — that  is, 
refuse  and  clean  coal — and  jigs  making  three  products  such  as 
clean  refuse,  middle  product  and  clean  coal;  or  refuse,  hutch 
work  and  clean  coal. 

Jigs  can  also  be  classified  according  to  the  means  used  to  pro- 
duce the  pulsation  of  the  water.  Thus  there  are  machines  where 
the  whole  jig  basket  is  moved  up  and  dowri,  and  jigs  with  sta- 
tionary screens  in  which  the  pulsation  of  the  water  is  produced 
either  by  a  plunger  or  by  means  of  compressed  air;  or  as  in  the 
Richard  pulsator  jig,  by  hydraulic  shocks.  The  plungers  can 
also  be  arranged  differently.  They  may  be -located  either  in  a 
separate  compartment,  which  again  can  be  placed  in  the  rear 
of  the  jig  compartment,  or  on-one  or  both  sides  of  the  screens. 
We  also  have  jigs  with  the  plunger  directly. ,  underneath  the 
screen.  One  type  of  jig,  with  the  plunger  placed  in  a  vertical 
position  below  the  screen  has,  however,  been  proved  a  failure. 

The  jig  screens  are  usually  made  of  perforated  steel  plate; 
castriron  grate  bars  are  also  sometimes  used.  The  method  of 
fastening  the  screen  plates  shows  numerous  variations.  Tlie 
methods  used  for  refuse  discharge  are  too  numerous  to  men- 
tion, but  they  can  be  broadly  divided  into  plain  slide  gates, 
either  adjustable  in  a  vertical  position  or  swinging  outward; 
double  gates,  in  which  the  lower  gate  regulates  the  height  of 
bed  and  the  upper  gate  regulates  the  outflow  of  refuse ;  revolving 
slate  valves ;  kettles  or  pot  valves  and,  finally,  discharge  of 
refuse  through  an  artificial  bed. 

Furthermore,  jigs  can  be  classified  as  one,  two  or  three  com- 

161 


162 


COAL  WASHING 


partment  machines.  If  we  consider  that  in  addition  to  the  dif- 
ferences mentioned  in  construction  jigs  can  be  built  either  of 
wood,  steel  plates,  cast-iron  plates  or  even  of  reinforced  con- 
crete, and  that  the  plungers  can  be  actuated  by  fixed  or  adjust- 
able eccentrics  or  by  means  of  crank-arm  mechanisms,  and  that 
each  single  type  of  each  group  can  be  used  without  great  changes 
in  any  other  group,  we  get  so  many  varieties  that  a  systematic 
classification  of  the  jigs  into  distinct  types  is  almost  impossible. 
By  considering,  however,  so  far  as  is  feasible  all  the  important 


Fig.  75.     Liihrig  Nut  Coal  Jig 

differences,  we  can  distinguish  the  following  three  general  types : 
Jigs  with  Fixed  Screens — These  include  (a)  coarse  coal  jigs, 
making  two  products  only — refuse  and  washed  coal.  The 
plungers  are  in  rear  of  the  jigs,  actuated  either  by  fixed  or  ad- 
justable eccentrics  or  by  a  crank-arm  mechanism  giving  a  differ- 
ential motion,  (b)  Fine  coal  jigs  with  an  artificial  bed.  These 
have  the  same  plunger  arrangement  and  drive  as  those  of 
class  (a).  Refuse  is  withdrawn  from  the  hutch  and  washed 
coal  overflows  at  the  front  of  the  jig.  (c)  Coarse  coal  jigs  with 
plungers  underneath  the  screens.  Either  eccentric  or  crank- 


TYPES  OF  JIGS 


163 


arm  mechanisms  are  employed  for  giving  the  plunger  motion. 
The  refuse  is  discharged  and  the  washed  coal  overflows,  (d) 
Jigs  with  plungers  on  both  sides  of  the  screen.  The  plunger 
motion  may  be  like  that  in  type  (a).  These  machines  have  either 
simple  refuse  and  washed  coal  discharge  or  a  third  discharge 
for  middle  product.  This  type  is  chiefly  constructed  as  two- 
or  three-compartment  jigs,  (e)  Jigs  with  one  plunger  between 
compartments.  This  type  is  hardly  ever  used,  (f)  Jigs  with- 
out plungers.  The  pulsation  is  actuated  by  puffs  of  compressed 
air  or  by  hydraulic  shocks. 


I s'-s-- 4- 

Fig.  76.     Forrester  Jig 

A  great  variety  of  jigs  with  fixed  screens,  treating  coarse 
(nut)  coal  were  developed  from  the  "Hartz"  type  of  jig.  The 
following  descriptions  and  illustrations  will  show  clearly  the 
most  typical  constructions. 

(1)  The  Liihrig  nut  coal  jig  is  a  single-compartment  ma- 
chine whose  plunger  is  given  a  simple  reciprocating  motion  by 
means  of  an  eccentric.  The  jig  of  this  type  shown  in  Fig.  75  con- 
sists of  a  rectangular  box  with  hopper  bottom  having  a  partition 
in  the  middle,  extending  about  half  way  down  from  the  top,  or  to 
a  point  slightly  above  where  the  hoppering  begins.  Upon  one 


164 


COAL  WASHING 


side  of  this  partition  is  a  relatively  close  fitting  rectangular 
plunger  actuated  by  an  eccentric.  On  the  other  side  of  the 
partition  there  is  a  fixed  screen  slightly  inclined  away  from  the 
partition.  The  jig  is  filled  with  water,  to  which  the  plunger 
imparts  a  pulsating  motion,  forcing  it  up  and  down  through  the 
screen.  Sized,  raw,  nut  coal  is  fed  upon  the  screen  near  the 
partition  and  purified  by  the  hindred  settling  action  induced  by 
the  pulsation  of  the  water  through  the  screen.  The  washed 


Fig.  77.     Nut  Coal  Jig  with  Double  Slate  Gate 

coal  flows  from  the  top  of  the  screen  compartment  at  the  oppo- 
site end  from  the  feed,  while  the  refuse  works  its  way  across, 
assisted  by  the  slope  of  the  screen,  and  the  excess  over  that  re- 
quired to  maintain  a  suitable  bed  is  discharged  through  a  gate 
just  above  the  screen  and  below  the  washed  coal  overflow.  The 
bed  is  kept  thin  enough  to  permit  regular  and  even  pulsations 
of  water  through  the  screen,  and  thick  enough  to  prevent  fine 
coal  from  working  through  by  the  aid  of  suction  and  entering 


TYPES  OF  JIGS 


165 


the  hoppered  bottom,  or  hutch,  of  the  jig.  The  refuse  which 
collects  in  the  hutch  is  discharged  at  intervals,  as  required, 
through  a  gate  at  the  bottom. 


Fig.  78.     Nut  Coal  Jig.     Top  View 


(2)     The  Forrester  jig  is  shown  in  Fig.  76.     This  jig  is  sim- 
ilar in  construction  to  the  Liihrig  machine.     The  plunger  is 


Fig.  79.     Adjustable  Eccentric 

actuated  by  means  of  a  crank  and  walking  beam.     The  drive 
shaft  is  located  below  the  jig  and  the  refuse  is  carried  away  by 


166 


COAL  WASHING 


means  of  a  screw  conveyor.  The  jig  screen  is  30  by  36  in.,  and 
is  perforated  with  Vis  in.  round  holes.  The  jig  has  a  rated  ca- 
pacity of  25  tons  of  2l/2  in.  screenings  per  hour.  The  plunger 
makes  6  in.  strokes  at  a  rate  of  40  per  minute. 

Figs.  77  and  78  show  a  nut  coal  jig  built  of  wood.  This  ma- 
chine is  used  extensively  in  Europe.  It  has  a  double  gate 
for  the  removal  of  the  refuse. 


Fig.  80.     Nut  Coal  Jig  Built  of  Steel  Plate 

Fig.  80  shows  a  nut  coal  jig  built  of  steel  plate. 

The  raw  coal  is  fed  on  to  the  jig  at  "A"  and  is  carried  for- 
ward in  the  direction  of  the  arrow.  The  clean  coal  overflows 
over  the  plate  "C."  The  refuse  is  discharged  at  "B"  and  its 
removal  can  be  regulated  by  the  gates  "bx  and  b2."  The 
refuse  from  several  jigs  is  collected  at  "g"  and  carried  by  a 
screw  conveyor  to  the  refuse  elevator  "H."  The  hutch  work 
drops  through  "F"  into  the  same  conveyor  as  the  refuse.  The 
wash  water  can  be  regulated  by  a  valve  "J."  The  plunger 


TYPES  OF  JIGS 


167 


is  actuated  by  an  adjustable  eccentric.     Fig.  79  shows  the  con- 
struction of  this  eccentric. 

Figs.  81,  82  and  83  show  a  nut  coal  jig  with  differential  mo- 
tion of  the  plunger  by  means  of  a  crank  arm  mechanism. 

This  mechanism  imparts  to  the  plunger  a  quick  and  sharp 
down  stroke  and  a  slow  up  stroke.  This  causes  a  lively  loosen- 
ing up  of  the  material  on  the  jig  screen  during  the  down  stroke 


Fig.  81.     Nut  Coal  Jig  with  Plunger  Actuated  by  a  Crank  Arm  Mechanism 

of  the  plunger  and  a  slow  settling  of  the  particles  during  the 
up  stroke.  The  operation  of  this  jig  is  greatly  simplified  by 
having  all  the  regulating  levers  close  together  and  within  easy 
reach  of  the  operator.  The  wash  water  can  be  regulated  by  the 
butterfly  valve  "g"  operated  by  the  pull  rod  "a."  The  lever 
"b"  regulates  the  refuse  discharge  and  the  lever  "f"  opens 
and  closes  the  gate  "d"  which  regulates  the  discharge  of  the 
hutch  work. 

The  Elmore  jig  is  the  outcome  of  17  years  of  development. 
It  has  grown  from  a  comparatively  small  machine  of  light  con- 


168 


COAL  WASHING 


struetion  to  the  largest  and  heaviest  coal  jigging  device  of  its 
type.  The  changes  that  have  been  wrought  have  all  been  the 
result  of  real  experience  with  all  kinds  and  conditions  of  coal. 
The  result  has  been  that  this  jig  will  stand  continuous  opera- 
tion under  the  heaviest  loads  and  will  automatically  make  uni- 
form products  of  clean  coal  and  clean  refuse. 

The  power  required  to  operate  this  jig  varies  somewhat,  de- 
pending on  the  size  of  coal  fed  to  it.  The  finer  sizes  require  a 
shorter  plunger  stroke,  hence  less  power.  The  power  required 
to  operate  the  jig  alone,  not  taking  into  account  any  provision 
for  transmission  machinery,  will  vary  from  10  h.p.  for  the 


Fig.  82.     Top  View  of  Jig 

finest  sizes  up  to  13  h.p.  for  3  in.  nut.     To  this  must  be  addt 
the  transmission  loss,  which  when  the  usual  main  and  counter- 
shafts are  used,  will  probably  be  from  2  to  3  h.p. 

If  direct  drive  can  be  provided,  it  has  been  found  that  silent 
chain  is  far  more  satisfactory  than  gears.  The  standard  jig 
is  equipped  with  a  pair  of  36x8  in.  tight  and  loose  pulleys  for 
driving  the  main  jig  shaft.  The  tight  pulley  weighs  approxi- 
mately 750  Ibs.,  and  has  a  balance  wheel  effect,  which  gives  an 
even  motion  to  the  plunger  stroke. 

The  "600-A"  coal  jig  (as  the  machine  shown  is  designated) 
consists  of  a  tank  with  a  partition  (48)  extending  part  way 
down  into  it.  (Fig.  84.)  On  one  side  of  this  partition  is, a 
plunger  (35)  which  is  given  a  reciprocating  motion.  On  the 
other  side  is  a  sieve  or  grate  (13).  In  front  of  the  grate  is  a 


TYPES  OF  JIGS 


169 


dam  (38)  over  which  all  water  and  coal  must  pass  when  they 
leave  the  jig,  flowing  away  on  the  overflow  plate  (25  Fig.  84  or 
(1)  Fig.  87).  The  tank  is  filled  with  water  up  to  the  level  of 
the  top  of  the  dam  (38)  and  covers  the  plunger  (35). 

Each  downward  stroke  of  the  plunger  will,  therefore,  force 
or  pump  a  volume  of  water  upward  through  the  sieve    (13) 


Fig.  83.     End  View  of  Jig 


equal  to  the  amount  displaced  by  the  plunger  stroke.  Raw 
coal  containing  pure  coal,  bone  and  the  heavier  refuse  is  fed 
to  the  jig  between  the  feed  plate  (37)  and  the  partition  (48, 
Fig.  84)  and  falls  on  to  the  sieve  (13).  The  pulsation  of  the 
stroke  will  now  draw  this  raw  coal  under  the  opening  at  the 
bottom  of  the  feed  plate  and  make  a  level  "bed"  all  over  the 
sieve.  This  will  continue  to  fill  up  until  the  top  of  the  dam 


170 


COAL  WASHING 


TYPES  OF  JIGS 


171 


172  COAL  WASHING 

(38)  is  reached,  when  both  water  and  clean  coal  will  flow  over 
on  plate  (25). 

While  this  filling  up  of  the  bed  has  been  taking  place,  the  pul- 
sations of  the  stroke  have  caused  the  light  coal  to  go  to  the  top 
of  the  bed  and  the  heavy  particles  of  refuse  to  settle  to  the 
lowest  strata  on  the  sieve  and  form  the  "bottom."  This  sep- 
arating process  continues  until  the  bottom  has  accumulated  to  a 
depth  equal  to  about  one-half  the  height  of  the  dam  (38)  ;  in 
other  words,  until  about  one-half  the  material  in  the  bed  is 
coal  or  light  and  the  other  half  refuse  or  heavy.  The  "bone" 
that  may  be  present  in  the  feed  will  arrange  itself  in  the  middle 
zone,  between  the  refuse  and  the  pure  coal.  A  constant  supply 
of  water  must  be  furnished  the  jig,  preferably  by  a  large  pipe 
entering  the  tank  under  the  plungers.  When  the  slate  or  refuse 
has  accumulated  to  the  thickness  above  mentioned,  all  further 
refuse  that  comes  into  the  jig  must  be  immediately  removed, 
otherwise  the  * '  bottom ' '  will  get  too  thick  and  refuse  will  go 
over  the  top  of  the  dam  with  the  washed  coal.  The,  means  by 
which  this  refuse  is  removed  will  be  fully  described  in  the  fol- 
lowing paragraphs. 

As  fast  as  the  refuse  is  withdrawn,  it  is  removed  from  the 
tank  by  the  drag  conveyor  (44).  Any  fine  refuse  that  may  go 
through  the  holes  in  the  sieve-  will  collect  in  the  "hutch"  of 
the  jig  (the  hopper  in  the  tank  under  the  plungers  and  the 
sieve).  These  fines  are  removed  from  time  to  time  by  opening 
the  slush  gate  (29). 

A  jig  tank  must  carry  not  only  the  pressure  of  the  water  that 
it  holds,  but  it  must  be  rigid  enough  to  carry  the  weight  of 
the  heavy  machinery  mounted  on  and  in  it,  as  well  as  the  added 
pressure  due  to  the  pulsations  of  the  plunger.  These  pulsations 
come  at  the  rate  of  approximately  100  -per  minute  and  unless 
the  walls  of  the  tank  are  made  quite  rigid,  they  will  "breathe" 
with  each  stroke  of  the  plunger.  This  will  quickly  destroy  the 
tank. 

The  best  type  of  jig  tank",  if  made  of  wood,  is  one  in  which 
the  walls  are  built  up  by  spiking  down  flatwise,  pieces  of  2  x  6  in. 
cypress  tank  stock,  surfaced  on  four  sides  to  5%  x  1%  in.,  letting 
all  corners  and  intersections  alternately  overlap,  and  placing 
two  strands  of  candle  wick  between  each  layer  and  in  all  verti- 


7 T /'/•>'  -OF 


173 


cal  edge  joints.  The  spikes,  not  smaller  than  20  d,  are  driven 
about  8  in.  apart,  staggered,  and  close  to  the  edges,  with  the 
candle  wick  running  between  the  two  rows. 

To  further  strengthen  the  tank,  vertical  rods,  some  of  them 
I1*  in.  in  diameter,  pass  through  from  the  top  to  the  bottom  of 
the  sills.  Others  pass  through  the  tank  horizontally  at  places 
which  receive  the  greatest  stress  from  the  pulsating  action  be- 


Fig.  80.     Front  View  of  Elmore  "600-A"  Coal  Jig 


fore  referred  to.  The  construction  of  the  tank  is  shown  in  Fig. 
86.  The  combined  effect  is  to  make  a  jig  tank  practically  non- 
rotting  (being  built  of  cypress)  and  as  rigid  as  a  concrete  wall. 
After  the  tank  has  been  filled  with  water  for  24  hr.,  all  leaks  will 
disappear. 

The  main  shaft  (3  Fig.  85)  carrying  the  eccentrics  which  pro- 
duce the  jigging  stroke,  is  47/ie  in.  in  diameter.     It  revolves  at 


174  COAL  WASHING 

100  r.p.m.  in  extra  heavy  bearings,  mounted  on  sole  plates  (4) 
of  special  heavy  design.  Having  in  mind  the  fact  that  the  work 
of  the  plungers  is  all  done  on  the  downward  stroke,  it  will  be 
seen  that  the  pressure  on  the  bearings  and  sole  plates  is  all  in 
the  upward  direction.  Hence  the  bolts  which  secure  these 
parts,  the  caps  to  the  bearings  and  the  castings,  must  be  much 
heavier  than  is  usually  required  for  shafting  of  this1  size.  All 
these  details  have  been  carefully  -worked  out. 

This  main  shaft  carries  two  eccentrics  (34  Fig.  84)  each  in- 
cased in  a  housing  (2).  These  eccentrics  act  in  unison  and  are 
fitted  in  such  a  manner  that  the  stroke  can  -be  adjusted  from 
zero  up  to  4  in.  The  eccentrics  are  connected  to  the  housing 
by  the  wrist  pins  at  the  lower  end  of  the  arms  on  the  under 
halves  of  the  eccentric  straps.  The  housings  are  bolted  to  the 
plungers  (35  Fig.  84)  and  by  a  system  of  self -oiling  internal 
guides,  are  given  a  true,  reciprocating  motion.  The  housings 
carry  a  quantity  of  oil  in  which  the  eccentric  straps  are  con- 
stantly submerged. 

Two  eccentrics,  each  acting  with  its  own  plunger,  are  used  for 
the  reason  that  one  plunger  would  be  too  long  and  heavy  for 
convenient  handling  and  stroke  adjustment.  It  will  be  noted 
that  the  wall  between  the  two  plungers  extends  down  into  the 
tank  only  far  enough  to  hold  the  rubbing  plates  (47),  which 
surround  the  plungers.  This  center  wall  (48)  also  affords  a  good 
foundation  for  the  center  shaft  bearing  with  its  sole  plate.  The 
eccentrics  and  housings  are  of  heavy  design. 

The  length  of  stroke  required  to  produce  the  best  jigging  ac- 
tion (the  supply  of  water  being  sufficient)  varies  with  the  size 
of  raw  coal  fed  to  the  jig.  With  coal  which  has  been  passed 
through  hammer  crushers  (as  is  frequently  done  when  it  is  to 
be  used  for  making  metallurgical  coke),  and  reduced  so  fine 
that  40  to  60  per  cent,  will  pass  through  an  %  in.  round  hole, 
the  stroke  is  not  more  than  %  in.,  for  nut,  it  will  vary  from  l1/^  to 
2  in.,  for  egg  from  2  to  2fa  in.,  and  for  material  which  will  pass 
through  a  5  or  5^  in.  round  hole,  the  stroke  will  approximate  3  in. 

The  plungers  (35  Fig.  84)  are  made  of  four  layers  of  2  in. 
oak  piank,  surfaced  on  all  sides.  Around  the  top  edge  on  all 
four  sides  is  bolted  a  heavy  casting  (46),  to  which  is  bolted  6-ply 
rubber  belting,  6  in.  wide,  and  extending  downward  and  out- 


TYPES  OF  JIGS  175 

ward  in  such  a  manner  that  the  lower  edge  of  the  belting  comes 
in  contact  with  the  rubbing  plates  (47).  Above  this  point  there 
is  ample  space  for  the  water  to  pass  freely  between  the  walls 
of  the  compartment  and  the  plunger.  This  piece  of  rubber 
belting,  therefore  becomes  a  flap  valve,  which  closes  on  the 
downward  stroke  and  opens  on  the  upward  stroke.  This  pro- 
duces almost  a  theoretically  perfect  jigging  effect  in  the  bed 
of  coal  and  refuse  resting  on  the  sieve  (13)  opposite  the 
plunger. 

On  the  downward  stroke  the  valve  closes  and  forces  all  the 
water  displaced  by  the  plunger,  up  through  the  bed,  where  it 
does  its  work  of  separating  the  coal  from  the  refuse.  With  a 
plunger  not  equipped  with  these  flap  valves,  much  of  the  water 
displaced  passes  up  around  all  four  sides  to  the  top  of  the 
plunger,  doing  no  work  of  separation.  Such  jigs  require  much 
longer  stroke. 

On  the  upward  stroke  of  the  plunger,  these  flap  valves  yield 
immediately,  and  break  all  tendency  to  form  a  vacuum  under 
the  plunger.  It  is  a  cardinal  principle  in  the  art  of  jigging, 
either  of  ore  or  coal,  that  suction  in  the  bed  of  the  jig  must  be 
avoided.  The  forming  of  a  vacuum  under  the  plunger,  on  the 
upward  stroke,  is  detrimental  to  the  jigging  action  and  has  a 
tendency  to  undo  the  good  work  performed  in  the  bed  while  the 
plunger  was  on  its  downward  stroke. 

By  referring  to  (13)  Fig.  84  and  (2)  Fig.  87  the  two  forms 
of  perforated  screens  held  rigidly  in  place,  on  which  the  bed  of 
coal  and  refuse  rests,  can  be  easily  seen. 

For  coarse  sizes  the  form  shown  in  Fig.  84  is  usually  employed. 
It  is  formed  of  cast-iron  grates  made  in  segments  12  in.  wide, 
and  securely  bolted  to  the  rests  which  extend  entirely  across 
the  width  (7  ft.)  of  the  jig  bed.  These  heavy  rests  are  bolted 
to  the  walls  of  the  jig  tank. 

For  fine  sizes  such  as  slack  the  construction  shown  in  Fig.  87 
is  used. 

The  sieve  (2),  which  is  of  perforated  plate  either  steel  or 
bronze  (depending  on  the  acid  in  the  water),  is  securely  bolted 
to  the  heavy  cast-iron  supports  (3)  which  extend  from  wall  to 
wall  and  rest  on  the  pieces  (9)  which  are  bolted  to  the  tank. 
The  holes  in  this  plate  are  usually  quite  small,  %  in.  round 


176 


COAL  WASHING 


being  the  average  size,  although.  %e  in.  and  }4  in.  are  some- 
times better.  The  size  of  these  holes  is  gaged  by  the  amount 
of  fines  in  the  refuse.  It  is  not  advisable  to  let  too  much  fines 
pass  through  these  holes  into  the  hutch  of  the  jig. 

Note  the  triangular  pieces  (36,  Fig.  84,  and  4,  Fig.  87),  under 
the  grate  or  sieve  at  the  back,  or  feed  side.  These  are  pieces 
of  6  x  6  in.  oak,  sawed  diagonally  from  corner  to  corner  and 
secured  by  bolts  at  the  ends  to  castings  which  in  turn  are  bolted 
to  the  tank  walls,  as  shown.  These  triangular  pieces  perform  an 


Note: 

Put  £  Permanent  Liner  under  Sheet  Steel  Liner 
H.I.J.K  then  put  on  Sheet  Liwsr  H,1,J,K,  then  bolt 
Feed  Gate  Guide 

^ ,        Note: 

~*  "/      /     Nail  on  liner  of  2  plank  this  entire  side 
above  sieve  and  run  boot  under  tLeet 
Iron  Liner  R.  fctcp  at  Sheet  Steel  Liner  H,I,J,K 


~1 


8iT 


-E 


Section  XX 
Fig.  87.     Arrangement  of  Jig  Screen  for  Fine  Coal 


important  service.  "When  the  plunger  makes  its  downward 
stroke  and  forces  the  water  up  through  the  perforated  grate 
or  sieve,  the  tendency  is  for  the  larger  portion  of  this  water  to 
take  the  path  of  least  resistance  and  go  up  through  the  back 
side  of  the  sieve,  near  the  feed  plate  (37,  Fig.  84,  or  6,  Fig.  87), 
leaving  the  front  end  of  the  bed  in  the  region  of  the  discharge 
for  both  coal  and  refuse,  with  little  jigging  action.  These 
triangular  pieces  act  as  a  resistance  to  the  free  flow  of  water 
upward  through  the  sieve  above  them,  and  force  a  proper  pro- 
portion of  the  water  toward  the  front  part  of  the  bed,  thus 
producing  uniform  jigging  action  over  the  whole  area  of  the  bed. 
Reference  to  1  Fig.  87  will  show  the  form  of  overflow  plate 
used  for  bituminous  coal  containing  slack  sizes.  This  plate  ex- 
tends across  the  7  ft.  width  of  the  jig,  and  gets  its  name  from 


TYPES  OF  JIGS  177 

the  fact  that  the  washed  coal  and  all  the  water  used  in  the  jig- 
ging process  flows  over  this  plate.  From  this  plate  they  both 
pass  to  the  main  washed  coal  settling  tank.  This  may  be  of 
various  kinds  but  is  a  part  of  every  properly  designed  coal 
washery.  The  upper  end  of  this  plate  is  bent  downward  to  a 
vertical  position  and  to  this  portion  is  bolted  the  adjustable 
plate  (8)  for  regulating  the  width  of  the  opening  underneath, 
where  the  refuse  passes  to  the  rotating  refuse  valve  (5).  The 
size  of  the  opening  does  not  in  any  way  regulate  the  amount  of 
refuse  which  is  discharged.  It  is  simply  set  wide  enough  to  per- 
mit the  easy  passage  of  any  piece  of  refuse  which  properly  can 
come  to  the  jig.  As  will  be  shown  the  amount  of  refuse  re- 
moved is  entirely  controlled  by  the  rate  of  rotation  of  the 
valve  (5). 

When  the  raw  coal  fed  to  the  jig,  is  a  sized  product  with  all 
the  slack  taken  out,  it  is  usual  to  employ  the  overflow  plate  con- 
struction shown  in  Fig.  84.  Here  two  plates  are  used,  one  (25) 
being  perforated  with  long  slot  holes  large  enough  to  allow  all 
the  water  to  drop  through  on  the  solid  plate  (28).  This  both 
dewaters  and  rinses  the  washed  coal  at  the  same  time.  If  it 
contains  any  small  slime  coal  as  a  result  of  bad  screening  or 
abrasion  in  the  bed,  this  is  washed  off  and  goes  with  the  water, 
leaving  the  sized,  washed  coal  clean  and  bright.  The  water 
which  falls  on  the  plate  (28)  passes  immediately  to  the  slack 
coal  dewatering  tank,  where  the  coal  is  removed  and  the  water 
re-used  in  the  jig. 

Various  types  of  equipment  are  used  to  remove  the  refuse 
from  the  bed  of  coal-jigging  machines.  They  are  divided  into 
two  classes :  The  pot  valve,  or  pocket  and  the  long  rotating 
valve  or  gate.  The  former  are  openings  from  12  to  24  inches 
apart  in  the  bed  of  the  jig  placed  near  the  discharge  end,  with 
devices  which  are  intended  to  prevent  the  coal  from  getting 
into  them.  Refuse  coming  into  the  jig  must  find  its  way  to 
these  pockets,  and  in  order  to  reach  them,  the  most  of  it  travels 
diagonally  to  the  flow  of  the  coal  and  the  water.  In  other 
words  the  opening  for  the  discharge  of  refuse  does  not  extend 
entirely  across  the  jig  bed  thereby  allowing  each  piece  of  refuse 
to  take  its  direct  and  natural  path  to  it,  but  each  piece  of 
refuse  must  find  the  special  holes,  located  many  inches  apart, 


178  COAL  WASHING 

through  which  it  can  escape.     Inert  refuse  remains  on  the  screen 
between  the  pot  valves  reducing  thereby  the  jigging  area. 

The  Elmore  Automatic  Jig  Control.  Beginning  as  a  rather 
complicated,  electrically  controlled  device,  the  Elmore  automatic 
control  as  now  built  as  shown  in  Figs.  88,  89  and  90.  Fig.  88 
shows  the  section  transversely  across  the  jigging  compartment, 
just  behind  the  overflow  plate  (14)  and  directly  through  the 
float  cylinder  (1).  Fig.  89  is  a  side  elevation  of  the  front  end 
of  the  jig,  showing  that  portion  of  the  tank  and  equipment  where 
the  overflow  plate,  the  automatic  control  device  and  the 
mechanism  for  actuating  the  refuse  valve  are  located.  Fig.  90 
shows  a  plan  of  the  jigging  compartment. 

To  the  overflow  front  casting  (14)  is  bolted  the  float  cylin- 
der (1).  This  is  10  in.  in  inside  diameter  and  22  in.  high,  open 
at  both  ends.  The  bottom  is  placed  3  or  4  in.  above  the  jigging 
sieve,  which  brings  it  below  the  level  of  the  top  layer  of  refuse 
(20)  which  it  is  desired  to  maintain  on  the  sieve.  On  top  of 
the  layer  of  slate,  will  accumulate  a  layer  of  clean  coal  (22) 
to  a  height  and  thickness  up  to  the  top  of  the  overflow  front 
(14)  where  the  clean  coal  with  the  water  used  in  jigging  will 
overflow  onto  the  plate  (15).  For  treating  slack,  this  is  made 
solid,  but  for  jigging  nut  or  coarser  sizes,  it  is  perforated,  al- 
lowing the  water  to  go  through  and  accumulate  on  plate  (16). 
Inside  the  float  cylinder  is  the  flat  cast  iron  disk  (21)  carried 
at  the  lower  end  of  the  pipe  guide  (2).  Attached  to  this  pipe 
guide  is  the  arm  (24)  which  at  the  other  end  is  fixed  to  the 
square  shaft  bar  (4),  carried  in  the  bearings  (3).  At  the  end 
of  this  shaft,  which  extends  out  over  the  wall  of  the  jig,  is  the 
double  arm  (5).  One  end  of  this  arm  carries  the  counterweight 
(10)  and  the  other  end  is  connected  with  the  rod  (6)  in  such 
a  manner  that  it  can  lift  the  pawl  (8).  This  pawl  is  a  part  of 
an  oscillating  mechanism  mounted  on  the  shaft  (11)  which  forms 
the  stem  of  a  rotating  valve  for  drawing  off  the  refuse.  This 
valve  extends  entirely  across  the  jigging  bed  which  in  this  par- 
ticular machine  is  7  ft.  wide. 

The  refuse  passes  through  the  opening  below  the  adjusting 
plate  (17)  and  lies  against  the  flights  of  this  rotating  feeder 
type  of  valve.  As  this  valve  rotates,  the  refuse  is  withdrawn 


TYPES  OF  JIGS 


179 


180  COAL  WASHING 

from  the  jigging  sieve  and  falls  on  a  sloping  board  in  the  com- 
partment underneath,  whence  it  runs  by  gravity  to  the  refuse 
drag  conveyor  which  removes  it  from  the  jigging  tank.  The 
constant  oscillating  motion  of  the  arm  (12)  is  given  by  the 
crank  (5)  and  connecting  rod  (6,  Fig.  85),  the  crank  being 
driven  from  the  main  jig  shaft  by  suitable  sprockets  and  chain. 
Keyed  to  the  valve  stem  (11)  is  the  ratchet  wheel  (9). 

It  is  evident  that  when  the  pawl  is  down  and  the  arm  (12)  is 
oscillating,  the  ratchet  wheel  (9)  is  rotated  in  the  direction  of 
the  arrow  as  shown,  and  that  when  the  pawl  (8)  is  raised  and 
not  allowed  to  engage  with  the  teeth,  no  rotation  of  this  ratchet 
will  take  place,  inasmuch  as  the  bars  (12)  simply  ride  on  the 
shaft  (11)  and  are  not  keyed  to  it. 

The  automatic  control  of  the  rotation  of  the  refuse  valve  is, 
therefore,  entirely  a  matter  of  permitting  the  pawl  to  engage  the 
teeth  and  rotate  the  valve  whenever  the  level  of  the  slate  or 
refuse  has  reached  the  proper  thickness  on  the  jigging  sieve. 
Conversely,  the  rotation  of  the  valve  and  the  removal  of  refuse 
ceases  when  the  level  of  the  bed  of  refuse  has  been  lowered  to 
the  desired  point.  This  is  readily  accomplished  by  the  equip- 
ment shown  in  the  following  manner: 

As  the  refuse  accumulates  on  the  jigging  sieve,  it  will  find 
its  way  up  into  the  float  cylinder  (1)  from  the  bottom  end  and 
will  impinge  on  the  bottom  side  of  the  float  disk  (21).  At  each 
stroke  of  the  jigging  plungers  this  float  disk  will  receive  a  cor- 
responding impact  from  the  refuse  as  it  is  driven  upward  from 
below,  and  the  whole  structure  of  the  float  disk,  the  pipe  guide, 
as  well  as  the  arm  and  counterweight  will  have  a  reciprocating 
movement  with  a  few  degrees  of  angular  rotation  of  the  shaft  (4). 
This  upward  and  downward  movement  is  conveyed  to  the 
(8)  which  readily  oscillates  on  the  bolt  holding  it  in  position 
tween  the  arms  (12).  By  adjusting  the  counterweight  (10! 
to  properly  balance  the  effect  of  the  upward  pressure  of  the 
slate  on  the  bottom  of  the  float  disk,  an  equilibrium  is  quickly 
produced  whereby  this  float  disk  is  maintained  at  a  certaii 
level  in  the  jigging  bed. 

Whenever  additional  slate  comes  on  the  jig,  the  float  dis 
will  be  raised  within  the  cylinder  and  this  will  lower  the  con- 
necting rod   (6)   and  permit  the  pawl  to  engage  the  teeth 


TYPES  OF  JIGS  181 

the  ratchet  (9)  instantly  rotating  the  valve  and  removing  the 
refuse  through  the  orifice  under  plate  (17).  This  lowers  the 
level  of  the  refuse  on  the  jigging  sieve  and  permits  the  float 
disk  to  sink  to  a  lower  level,  when  the  pawl  will  again  be  lifted 
to  a  height  not  permitting  it  to  engage  the  teeth  of  the  ratchet. 
The  withdrawal  of  refuse  will  thus  be  instantly  stopped.  When 
nothing  but  clean  coal  comes  on  the  jig,  or  when  the  feed  is 
entirely  stopped,  the  pawl  will  accordingly  be  entirely  disen- 
gaged and  no  refuse  will  be  removed.  This  condition  is  not 
temporary,  but  will  continue  no  matter  how  long  the  jig  is  run. 

It  is  remarkable  how  accurately  this  simple  device  controls 
the  level  of  the  top  of  the  refuse  bed  in  the  jig.  No  matter 
how  the  rate  of  feed  may  vary,  either  in  tonnage  or  quality,  the 
level  top  of  this  slate  bed  is  maintained  almost  at  a  given  point 
throughout  the  entire  shift,  making  it  impossible  for  clean  coal 
to  find  its  way  through  the  orifice  under  plate  (17)  or  for 
refuse  to  accumulate  on  the  jigging  sieve  to  a  height  sufficient 
to  go  over  the  overflow  plate  at  (14).  Uniform  products  of 
clean  coal  and  clean  refuse  are  thus  secured. 

Formerly  the  jig  operator  determined  the  conditions  in  the 
jigging  bed  by  running  his  arm  down  into  the  coal  in  order 
to  find  the  top  of  the  bed  of  slate.  Another  method  was  to  use 
a  device,  such  as  a  bottle  on  the  end  of  a  broomstick,  and,  by 
practice,  learn  to  determine  this  level  by  the  "feel"  as  the  bottle 
was  lowered  through  the  coal  while  the  machine  was  in 
operation. 

Frequently  neither  of  these  tests  was  faithfully  practiced, 
and  the  regulation  of  the  machine,  insofar  as  the  refuse  re- 
moval was  concerned,  was  controlled  by  the  operator's  observa- 
tion of  the  two  products  coming  from  it.  Whenever  slate  was 
noticed  coming  with  the  coal,  more  refuse  was  withdrawn.  When- 
ever coal  was  noticed  coming  through  with  the  slate,  less  refuse 
was  withdrawn,  and  hence  the  adjustment  of  the  machine  was 
sometimes  permitted  to  become  entirely  wrong  before  steps  were 
taken  to  make  it  right. 

Jigs  with  an  Artificial  Bed.  This  type  is  used  only  for  fine 
coal,  and  the  refuse  is  discharged  through  the  perforations  of 
the  screen  into  the  hutch.  The  difficulties  of  jigging  increase 
with  the  fineness  of  the  coal,  The  artificial  bed  is  used  to  per- 


182 


COAL  WASHING 


TYPES  OF  JIGS  183 

mit  the  discharge  of  the  refuse  through  the  screen.  A  refuse 
discharge  through  a  slotted  opening  or  a  gate  would  result  in 
either  too  great  a  loss  of  good  coal  in  the  refuse  or  too  much 
refuse  would  be  carried  over  with  the  washed  coal.  Also,  the 
small  perforations  in  the  screen  required  by  the  fineness  of  the 
materials  would  clog  up  easily  and  thereby  nullify  the  pul- 
sations. 

In  order  to  avoid  all  this  trouble  a  screen  is  used  with  per- 
forations somewhat  larger  than  the  largest  size  of  material  to 
be  jigged.  On  top  of  this  screen  an  artificial  bed  is  laid,  the 
material  of  which  has  a  greater  specific  gravity  than  the  refuse. 
The  refuse  finds  its  way  through  the  interstices  of  the  bed  and 
drops  into  the  hutch.  The  best  material  for  an  artificial  bed 
has  been  found  to  be  feldspar.  For  the  first  compartment  feld- 
spar between  Bi  and  21A  in.  in  size,  and  for  the  second  compart- 
ment pieces  that  have  passed  through  a  B4  in.  and  over  %  in. 
round  holes,  is  used.  In  some  instances  iron  punchings  or  heavy 
slate  have  been  used  for  a  bed. 

The  discharge  of  the  refuse  is  regulated  in  addition  to  the 
proper  adjustment  of  the  plunger  stroke  and  speed,  by  the 
correct  thickness  of  the  bed  and  by  using  feldspar  of  a  proper 
size.  Both  these  values  can  only  be  determined  by  experiment. 
A  deep  bed  consisting  of  small-size  feldspar  will  give  cleaner 
refuse  than  a  shallow  bed  made  up  of  larger  pieces. 

Feldspar  is  especially  well  adapted  for  the  purpose  of  making 
an  artificial  bed  on  account  of  its  specific  gravity,  which  lies  be- 
tween 2.5  and  2.6.  On  account  of  its  hardness  it  resists  abrasion, 
and  being  sharp-cornered  makes  a  safe  bed,  permitting  the  small 
particles  of  refuse  to  pass  through  but  keeping  back  the  larger 
pieces  of  good  coal.  Slate  being  soft,  wears  off  its  sharp  cor- 
ners too  easily  and  must  be  renewed  frequently.  Iron  is  too 
heavy  a  material  and  kills  or  at  least  weakens  the  pulsation  of 
the  water. 

Fine-coal  jigs  are  often  arranged  in  tandem,  forming  in  reality 
a  two-compartment  jig.  This  arrangement  exposes  the  material 
to  the  jigging  action  during  a  longer  period,  but  restricts  the 
capacity.  Fine-coal  jigs  show  the  same  variations  in  regard  to 
design,  operating  mechanism  and  materials  used  in  their  con- 
struction as  the  coarse  coal  jigs.  Fig.  92  shows  a  fine-coal  jig, 


184 


COAL  WASHING 


TYPES  OF  JIGS 


185 


186 


COAL  WASHING 


with  feldspar  bed,  built  of  timber,  as  used  by  the  Link  Belt 
Company. 
Fig.  93  shows  a  feldspar  jig  built  of  cast-iron  plates. 

Jigs  with  Plunger  Placed  Underneath  the  Screen.     This  type 
of  jig  occupies  less  room  for  the  same  screen  area.     The  effect  of 


Fig.  94.     Twin  Two-Compartment  Jig  with  Plungers  Underneath  the 

Screens 

the  water  pulsation  is  uniform  over  the  whole  screen  area  and 
they  require  less  power  on  account  of  the  reduced  friction  of  the 
water  against  the  tank  walls. 

A  double  two-compartment  jig  is  shown  in  Figs.  94  -and  95. 
Each  jig  has  two  screen  compartments  "a"  and  "b."     The  coal 


TYPES  OF  JIGS 


187 


enters  the  first  compartment  "a"  through  the  chute  "A"  and 
the  clean  coal  overflows  over  the  plate  "c."  A  part  of  the 
refuse  (the  heaviest)  is  discharged  at  the  end  of  the  screen  "a" 
through  an  adjustable  gate  "d"  and  the  remaining  portion 
leaves  the  jig  at  the  end  of  the  screen  "b"  through  a  similar 
gate  "e."  The  refuse  drops  through  the  spouts  "g"  into  the 
boots  of  the  refuse  elevators  "f."  Underneath  each  screen,  a 
plunger  suspended  by  two  plunger  rods  "1,"  is  located.  These 
plungers  have  an  inclined  top  surface,  so  that  the  refuse  can 


Fig.  95.     Top  View  of  Twin  Two-Compartment  Jig 

slide  off.  This  refuse  drops  into  an  inclined  spout  "k"  and 
enters  the  elevator  boots  "f."  The  plungers  of  the  screens  "a" 
are  connected  by  a  walking  beam  "m"  and  eccentric  drive  "n" 
to  one  unit  and  the  screens  "b"  are  driven  in  unison  by  the  ec- 
centric "o."  The  eccentrics  are  set  at  180  deg.  so  that  but  little 
power  is  required. 

The  Montgomery  jig  has  a  plunger  underneath  the  screen,  but 
the  plunger  is  flat  and  has  a  number  of  flap  valves  to  eliminate 
the  suction  on  the  down  stroke  of  the  plunger. 

Raw  coal  enters  behind  the  baffle  plate  "T"  and  passing 
under  it,  rises  to  the  level  of  the  overflow  "Q"  where  it  passes 


188 


COAL  WASHING 


out  with  water  into  the  settling  tank.  Refuse  is  drawn  at  "I" 
into  the  refuse  compartment  "J"  and  down  chutes  "J2"  to 
the  refuse  elevator.  Water  returns  from  the  settling  tank  en- 
ters the  jig  tank  at  "S"  and  on  the  upward  stroke  of  the  plunger 
or  piston,  enters  through  valves  "0"  into  the  space  under  the 
plunger.  As  the  plunger  moves  down  the  valves  "0"  open  and 
the  space  between  plunger  "N"  and  screen  "P,"  is  filled  with 


Fig.  96.     Montgomery  Jig 

water.  This  is  forced  upward  by  the  up  stroke  through  screen 
and  bed  of  coal. 

Fig.  97  shows  a  Montgomery  jig  with  a  different  driving 
mechanism  and  plunger  construction. 

Jigs  with  plungers  on  both  sides  of  the  screen  compartment, 
are  mostly  built  as  two-  or  three-  compartment  jigs  and  were 
developed  from  the  Faust  jig  which  originated  in  the  Joplin 
district. 

Figs.  98  and  99  show  the  Faust  jig  as  built  for  coal  washing 
by  the  Link-Belt  Co. 

This  jig  has  three  compartments,  the  screen  in  each  being 
slightly  lower  than  that  in  the  preceding  one.  Each  of  the 


TYPES  OF  JIGS 


189 


SLATI  ELEVATOR 


RAY  COAL  rco 
JN  HCRC 


PLUNGER  BDTTCM 

FLUNEER  VALVC3 


HUTCH  VALVES 


HUTCH  CONVEYOR 


CRQ99  SECTION  OF  MONTGOMERY  TYPE  '8'  CQAL 

Fig-  97 


190 


COAL  WASHING 


screens  is  supplied  with  a  pulsating  current  of  water  by  a  pair 
of  plungers,  one  on  each  side.  The  jig  is  intended  for  the 
smaller  sizes  of  unsized  coal  from  %  in.  down.  The  finer  refuse 


<///"////////w 

Fig.  98.     Faust  Jig.     Cross  Section 

passes  through  the  screens  into  the  hutch,  while  the  coarse  refuse 
is  removed  through  kettle  valves,  two  in  each  screen.  (A  typical 
kettle  valve  design  is  shown  in  Fig.  100.)  For  %  in.  unsized 
coal  the  plungers  make  130  strokes  per  minute.  The  stroke  in 


Fig.  99.     Faust  Jig.     Longitudinal  Section 

the  first  compartment  is  1  in.  and  in  the  second  compartment 
7/8  in. 

Figs.  101  and  102  give  the  construction  of  a  three-compart- 
ment, double-phmger  jig  built  by  the  Roberts  &  Schaefer  Co. 


TYPES  OF  JIGS 


191 


This  jig  has  kettle  valves  in  the  first  two  compartments  and 
a  rotary  valve  in  the  third  compartment  for  the  removal  of 
the  refuse. 

Figs.  103-104  show  a  two-compartment,  double-plunger  jig 
built  by  the  American  Coal  Washer  Co. 

Both  compartments  have  kettle  valves  for  the  removal  of  the 
refuse.  Each  plunger  has  8  poppet  valves,  which  open  on  the 
up-stroke,  for  the  purpose  of  killing  the  suction  under  the 
screens.  The  driving  mechanism  is  located  below  the  jigs,  mak- 


Fig.  100.     Typical  Design  of  a  Kettle  Valve 

ing  these  machines  accessible  from  all  sides.  The  jigs  are  built 
of  cast-iron  plates,  securely  bolted  together  and  stiffened  with 
heavy  ribs.  This  makes  a  durable  and  extremely  rigid  con- 
struction. 

Jigs  with  the  Plunger  Between  the  Compartments.  This 
type  of  jig  is  not  extensively  used.  To  get  the  proper  pulsa- 
tion a  large  plunger  would  be  required,  since  under  ordinary 
conditions  the  plunger  area  is  about  one  half  of  the  screen 
area.  For  this  reason  this  type  of  jig  is  mostly  employed  for 
treating  fine  coal  using  a  feldspar  bed  on  the  screen.  Fine 
coal  requires  a  less  intensity  of  pulsation  than  the  coarser  sizes 
and  a  relatively  smaller  plunger  area  will  be  sufficient. 

Figs.  105  and  106  show  a  two-compartment  double  jig  with 
plungers  between  the  screen  compartments. 

The  plungers  move  in  a  vertical  direction  and  separate  plun- 


192 


COAL  WASHING 


TYPES  OF  JWS 


193 


t(J 

K  to 

^ 


a 


^ 


194 


COAL  WASHING 


gers  are  used  for  the  first  and  the  second  compartment.  This 
permits  giving  the  plunger  for  the  second  compartment  a  differ- 
ent stroke  from  that  for  the  first  compartment.  This  is  neces- 


Fig.  105.     Two-Compartment  Double  Jig.     Top  View 

sary,  since  the  material  treated  in  the  second  compartment  is 
quite  different  from  that  coming  into  the  first  compartment, 
having  lost  most  of  its  heavy  impurities. 


Fig.  106.     Two-Compartment  Double  Jig.     Cross  Section 

Figs.  107  and  108  show  a  two-compartment  single  jig,  in 
which  the  plunger  is  located  below  the  screens,  and  moves  in  a 
horizontal  direction. 

This  arrangement  gives  the  same  pulsation  to  both  compart- 
ments, which  is  theoretically  wrong.  It  makes  a  decidedly  com- 


TYPES  OF  JIGS 


195 


pact  construction  with  all  the  moving  parts  out  of  the  way  of 
the  operator.  A  similar  arrangement  was  used  in  the  early  days 
of  ore  dressing.  The  three  compartment  double  jig  of  Kassel- 
owsky  shown  in  Figs.  109  and  110  has  a  vertical  plunger  under- 


Figs.  107  and  108.     Two  Compartment  Jig  with  Plunger  Below  the  Screens 

neath  each  compartment,  serving  two  screens.  In  this  construc- 
tion the  stroke  of  each  plunger  can  be  adjusted  according  to  the 
characteristics  of  the  material  on  each  screen.  This  type  of  jig 
has  been  later  on  adopted  by  Parsons  and  Fischer. 


196 


COAL  WAUHINQ 


TYPES  OF  JIGS  197 

Jigs  Without  Plungers.  Jigs  without  plungers  have  been  in- 
troduced by  Baum,  who  also  originated  the  slogan  "First  Wash- 
ing, then  Classifying." 

Figs.  Ill,  112  and  113  show  the  construction  of  a  two-com- 
partment jig  for  treating  unsized  coal. 

This  machine  makes  three  products,  and  the  pulsation  of  the 
water  is  produced  by  compressed  air  of  l1/^  to  2  Ib.  pressure  per 
square  inch.  This  reduces  the  moving  parts  to  simple  air  valves 
located  above  the  rear  or  water  compartment  of  the  jig.  The 
construction  of  the  air  valve  is  shown  in  Fig.  111.  Here  P  is 


Fig.  111.     Air  Valve  for  Baum  Jig 

the  inlet  port  for  the  compressed  air.  The  piston  valve  V  is 
moved  up  and  down  by  the  eccentric  drive  c.  In  the  highest 
position  of  the  valve,  as  shown  in  the  illustration,  the  air  in  the 
water  compartment  &  can  escape  through  the  exhaust  ports  R. 
In  the  lowest  position  of  the  valve  the  compressed  air  enters 
through  the  port  P  and  expands  at  first  to  some  extent  an  ac- 
count of  the  increase  of  area ;  but  soon  the  pressure  in  the  valve 
will  be  equal  to  the  pressure  in  the  supply  pipe.  The  motion  of 
the  water  is  reversed  on  the  exhaust  stroke  of  the  valve.  The 
amount  of  air  can  be  regulated  by  the  length  of  stroke,  the  in- 
tensity of  the  pulsation  by  the  pressure  of  the  air,  and  the  fre- 


198 


COAL  WASHING 


quency  of  the  pulsations  by  the  speed  of  the  eccentric  shaft. 
The  jig  shown  in  Figs.  112  and  113  has  two  screens,  d  and  e, 
both  of  which  are  slightly  inclined  against  the  direction  of 
the  flow  of  materials.  The  first  and  larger  screen  has  three  and 
the  second  or  smaller  one  two  air  valves.  The  heavy  slate  is  dis- 
charged immediately  at  the  feed  end  of  the  jig  through  the 
slate  gate  /  and  falls  through  the  chute  g  into  the  eleva- 
tor h.  This  peculiar  method  of  slate  discharge  prevents  the 


Fig.  112 
Longitudinal  Section 


Fig.  113 
Cross  Section 


Bautn  Jig 


disseminating  of  the  soft  and  triturable  refuse  with  the  coal,' 
and  also  prevents  the  choking  up  of  the  screen  with  heavy  slate. 
On  the  second  screen  e  the  clean  coal  is  separated  and  over- 
flows at  i.  Light  refuse  or  the  middle  product,  according 
to  the  character  of  the  coal,  is  discharged  at  k  and  falls 
through  the  chute  I  into  the  elevator  pit  m.  The  sludge 
and  fine  refuse  which  passes  through  the  perforation  of  the 
screens  is  conveyed  by  means  of  a  right-  and  left-hand  conveyor 
nl  and  n2  to  the  elevator  pits  h  and  m.  The  water  is  regulated 
at  p. 

No  fundamental  difference  exists  between  the  method  of  op- 
eration of  the  jig  described  above  and  other  machines.     The  pe- 


TYPES  OF  JIGS 


199 


culiarity  of  the  use  of  compressed  air  can  be  partly  explained 
by  the  large  size  in  which  the  jigs  are  built.  But  plunger-jigs 
also  are  built  in  equally  large  sizes  for  the  treatment  of  un- 
sized coal  and  are  quite  successful. 

Fig.  114  shows  a  single-compartment  "Baum"  jig  with  double 
slate  gate. 

The  above  described  Baum  jigs  have  the  great  advantage,'  that 
the  pulsation  of  the  water  can  be  regulated  by  the  intensity  of 
the  air  pressure,  without  interrupting  the  operation  of  the  jigs. 


Fig.  114.     Single  Compartment  Baum  Jig 

Reciprocating  Jigs  or  Jigs  with  Movable  Screen.  The  con- 
struction of  this  type  of  jig  does  not  show  so  many  varieties  as 
that  of  the  fixed-screen  type.  Jigs  of  this  type  are  used  for  un- 
sized coal  and  make  but  two  products.  The  only  difference  in 
the  construction  of  the  jigs  is  found  in  the  arrangement  of  the 
slate  gate  and  the  methods  used  to  diminish  the  suction  on  the 
upstroke.  Fig.  115  shows  the  Stewart  jig  in  connection  with 
the  jig  tank.  Fig.  116  shows  the  side  view  and  Fig.  117  the  top 
view,  or  rather  bird's-eye  view,  of  the  jig  basket.  Fig.  118 
shows  the  "American"  jig.  This  machine  is  provided  with  ad- 
justable wearing  plates  on  the  sides,  securing  a  water-tight  joint 


200 


COAL  WASHING- 


between  the  jig  basket  and  the  tank.  The  swinging  slate  gate, 
operated  by  the  reciprocating  motion  of  the  jig  basket,  is  opened 
at  intervals  but  always  to  its  fullest  extent,  in  order  to  prevent 
the  accumulation  and  jamming  of  large  pieces  of  refuse  in  front 
of  the  gate.  The  interval  between  successive  openings  of  the 


Fig.  115.     Stewart  Jig  with  Jig  Tank 

gate  can  be  changed  at  the  discretion  of  the  operator.  To  ob- 
tain a  closer  regulation  of  the  slate  discharge,  the  operator  can 
easily  regulate  the  length  of  time  the  slate  gate  remains  open. 
This  is  accomplished  by  means  of  a  sliding  cam  which  changes 
the  time  that  the  slate  gate  remains  open  by  infinitesimal  incre- 
ments. 
The  Pittsburg  Jig  is  shown  in  Fig,  120, 


TY1>E8  OF  JIGS  201 

This  Jig  is  of  the  well-known  reciprocating  type,  4  ft.  wide, 
and  7  ft.  long,  inside  dimensions,  with  the  new  feature  of  a 
secondary  bottom  fitted  with  flap  valves  so  arranged  as  to  admit 


Fig.  116.     Side  View  of  Stewart  Jig  Box 

of  a  free  upflow  of  water  on  the  downward  stroke  and  closing 
and  holding  the  water  on  the  upstroke  of  the  jig,  as  will  be 
readily  seen  by  the  accompanying  cut  of  the  jig ;  the  bottom  of 
the  jig  slopes  down  toward  the  front. 


Fig.  117.     Top  View  of  Stewart  Jig 

The  driving  mechanism  of  these  jigs  is  especially  interesting. 
These  jigs  travel  twice  as  fast  on  the  downstroke  as  they  do  on 
the  upstroke.  This  result  is  obtained  by  the  use  of  a  slotted 


202 


COAL  WASHING 


TYPES  OF  JIGS 


203 


lever  and  crank  fitted  with  hardened  steel  roller  and  brass  bush- 
ings, so  arranged  that  on  two-thirds  of  the  revolution  of  the 
driving  gear  wheels,  the  slotted  lever  is  pulling  the  jig  up  and 
on  the  remaining  one-third  of  the  revolution,  the  slotted  lever 
is  pushing  the  jig  down,  accomplishing  the  desired  results.  All 
this  mechanism  is  adjustable  as  to  length  of  stroke  and  differen- 
tial between  up  and  down  strokes. 

The  tanks  in  which  the  jigs  reciprocate  are  equipped  with  a 
flap  valve  which  allows  the  water  to  enter,  but  closes  as  soon 


Fig.   119.     American  Jig  Showing  Slate  Gate  Operating  Mechanism 


as  the  jig  begins  its  downstroke.  As  the  jig  begins  its  downward 
motion,  the  water  is  forced  up  through  the  valves,  and,  accord- 
ingly, up  through  the  screen  bottom,  lifting  off  the  material  from 
the  screen.  On  the  beginning  of  the  upward  stroke  the  flap 
valves  in  the  secondary  bottom  close,  permitting  the  materials  in 
the  jig  to  settle  and  stratify  in  a  practically  quiet  body  of 
water.  The  refuse  settles  to  the  bottom  and  gradually  works 
forward  with  each  stroke  to  the  slate  gate.  The  coal  flows  over 
with  the  water  on  to  a  very  finely  perforated  screen,  from  which 
it  is  conveyed  into  a  washed  coal  bin,  the  water  draining  through 


204 


COAL  WASHING 


STEEL  ROLLER 
WITH    BRASS 
BUSHING 


CAST  STEEL  LEVER 

LEVER  FOR 
OPERATING 
SLATE 
GATE 


Fig.  120.     Pittsburg  Jig 


Fig.  121.     Shannon  Jig 


TYPES  OF  JIGS  205 

into  a  supply  tank  which  adjoins  the  jig,  and  is  connected  to  it 
by  the  above  mentioned  flap  gate,  thereby  enabling  the  use  of 
the  same  water  a  number  of  times  without  the  necessity  of  pump- 
ing, each  jig  handling  its  water  automatically. 

The  Shannon  Jig,  shown  in  Fig.  121 l  does  not  attempt  to 
force  the  water  through  the  coal  in  the  pan  by  making  it  fit  the 
pan  compartment  snugly,  but  obtains  a  similar  effect  by  pro- 
longing the  sides  of  the  pan  downwards  below  the  screen  and 
using  comparatively  rapid  strokes.  The  pan  is  held  free  from 
the  walls  of  the  pan  compartment  by  means  of  rollers.  Another 
peculiarity  of  the  Shannon  jig  is  that  the  washed  coal  and 
pan  compartments  are  connected  at  the  top  so  that  water  is 
free  to  move  back  and  forth  between  them.  This  greatly  re- 
duces the  quantity  of  water  which  must  be  pumped  to  keep 
the  jig  going. 

The  jig  is  4  ft.  wide  by  6  ft.  long,  inside  dimensions,  and  is 
rated  to  treat  on  an  average  47  tons  of  raw  coal  per  hour.  The 
number  of  strokes  varies  from  72  to  90  per  minute,  while  the 
length  of  the  stroke  is  from  3  to  3%  in. 

i  "Coal  Washing  in  Illinois,"  by  F.  C.  Lincoln.  Bulletin  No.  69,  Engi- 
neering Experiment  Station,  University  of  Illinois. 


CHAPTER  XX 
GENERAL  DATA  ON  JIGS 

Owing  to  the  great  variety  in  the  construction  and  arrange- 
ment of  jigs,  it  is  hardly  possible  to  give  authentic  figures  in 
regard  to  the  work  performed.  This  furthermore  becomes  more 
complicated,  as  the  work  and  the  output  of  a  jig  depend  not 
only  on  the  proper  selection  of  certain  dimensions  such  as  length 
and  width  of  jig  screens,  proportion  between  plunger  and  screen 
area,  height  of  overflow  above  screen,  and  the  size  of  perfora- 
tions in  the  screen,  but  also  on  the  proper  regulation  of  the 
plunger  stroke,  speed  of  eccentric  shaft,  size  of  materials,  thick- 
ness of  bed,  water  supply  and  many  other  considerations. 

The  details  of  construction  and  the  methods  for  securing  a 
proper  adjustment  were  developed  by  each  builder  of  jigs  inde- 
pendently and  according  to  his  own  ideas.  For  these  reasons 
we  have  a  multiplicity  of  designs  and  data  on  operation  which, 
however,  give  similar  final  results.  In  any  case,  however,  the 
tabulation  of  the  most  important  data  for  each  type  of  jig  (see 
table  32)  requires  some  explanation. 

The  methods  used  will  change  the  relation  between  the-  differ- 
ent factors  independently  of  the  capacity,  which  naturally  in- 
fluences the  size  of  the  jig.  If  coal  is  sized  before  washing,  a 
series  of  small  jigs  will  be  required;  whereas  if  unsized  coal  is 
to  be  treated,  quite  frequently  one  large  coarse-coal  jig  will  suf- 
fice. This  will  explain  the  considerable  difference  in  the  di- 
mensions and  the  capacity  of  the  jigs.  The  character  and  the 
size  of  coal  on  the  one  hand  and  the  type  of  the  jig  on  the  other 
strongly  influence  the  length  of  the  plunger  stroke.  The  num- 
ber of  strokes  per  minute  must  be  increased  in  inverse  propor- 
tion, and  the  length  of  the  stroke  in  direct  proportion  to  the 
size  of  the  coal.  That  is,  coarse  coal  requires  a  slower  and  longer 
stroke  than  fine  coal,  for  which  a  short,  quick  pulsation  is  more 
advisable.  The  size  of  the  perforation  in  the  screen  must  be 

206 


JIG  DATA  207 

smaller  than  the  smallest  size  of  the  coal.  The  big  difference 
in  the  power  consumption  can  be  explained  by  the  different  sizes 
of  the  jigs. 

Jigs  for  unsized  coal  are  mainly  built  for  great  capacity.  In 
some  cases  the  whole  amount  of  raw-coal  screenings  is  put  over 
one  jig  only,  and  I  know  of  some  machines  treating  150  tons  of 
raw  coal  per  hour.  The  perforations  in  the  screen  are  larger 
than  in  the  screens  of  coarse-coal  jigs,  because  a  heavier  pulsa- 
tion is  required  to  loosen  up  the  unsized  coal.  This,  however, 
will  make  the  size  of  perforation  in  the  screen  larger  than  the 
size  of  the  smallest  particles  of  the  material  to  be  treated,  and 
the  refuse  collecting  on  the  screen  must  prevent  the  downward 
passage  of  the  fine  coal. 

In  addition  to  the  foregoing  considerations  it  is  of  great  im- 
portance to  determine  the  correct  thickness  of  the  artificial  bed 
and  the  proper  size  of  the  material  to  be  employed.  The  data 
shown  in  the  table  can  be  used  only  as  a  guide.  Different  coals 
will  require  different  beds. 

TABLE  OF  THE  MOST  IMPORTANT  JIG  DATA 

Coarse    Coal  Unsized    Coal  Fine  Coal 

Jigs  Jigs  Jigs 

Width  of  jig  screen    24  in.  to  6  ft.  6  in.    30  in.  to  6  ft.  6  in.    20  in.  to  7  ft.  3  in. 

Length        of        jig  6  ft.  6  in.  to  19  ft.    6  ft.  6  in.  to  21  ft. 

screen     3  ft.  3  in.  to  16  ft.  6  in.  4  in. 

Capacity  per  hour 

in  tons 5  to  150  30  to  150  5  to  70 

Capacity  per  sq.  ft. 
of  screen  area  in 
tons  0.4  to  1.0  0.8  to  1.5  0.3  to  0.5 

Xumber  of  strokes 

per  minute    20  to   100  35  to  110  100  to  200 

Length  of  stroke  in  • 

inches 1^2  to  16  1%  to  6  %  to  2 

Proportion  of  area 
of  plunger  to 
screen  area  0.7  :  1  to  1 :  3  1 :  2  to  1 :  2,5  1:1  to  1:1.5 

Size  of  perfora- 
tions in  screen  in 
inches %  to  %  in.  %z  to  %  %  to  Vi 

Size  of  bed  mate- 
rial in  inches %6  to  \Vz 

Thickness  of  bed  in 

inches 2  to  4 

Horsepower  re- 
quired    1  to  6  3  to  10  1  to  5 

TABLE  32 


208  COAL  WASHING 

THE  CONTROL  OF  THE  WASHING  PROCESS 

Upon  the  character  and  nature  of  the  raw  coal  depends  the 
selection  of  the  proper  type  of  jig,  as  well  as  the  methods  to  be 
followed  in  washing  the  coal,  but  the  efficiency  of  these  methods 
depends  much  upon  the  control  of  the  washing  process.  J.  T. 
Drakeley,  in  his  paper,  "Coal  Washing;  A  Scientific  Study," 
has  ably  analyzed  the  different  methods  of  control  and  the  effects 
that  changes  in  operation  of  the  jigs  have  upon  the  output. 

Considerable  more  attention  ought  to  be  paid  to  the  control 
of  coal  washeries.  Undoubtedly  at  many  washeries  regular  float 
and  sink  tests  are  not  made,  and  it  is  only  upon  the  interpreta- 
tion of  the  results  of  such  tests  that  it  is  possible  to  base  argu- 
ments in  favor  of  any  variation  in  the  working  of  the  washers. 
That  these  tests  should  be  made  is  recognized  generally,  espe- 
cially where  jig-washers  are  used,  as  their  control  introduces 
many  factors.  With  trough  and  inverted-cone  washers  the  free- 
dom of  control  is  more  restricted. 

The  control  which  is  exercised  over  jig  washeries  usually  con- 
sists in  regulating:  (1)  the  supply  of  raw  coal,  (2)  the  supply 
of  water,  (3)  the  stroke  and  speed  of  the  eccentric  action,  and 
(4)  the  outflow  of  the  refuse. 

(1)  Supply  of  Raw  Coal.     If  the  raw  coal  is  fed  too  rapidly 
onto  the  wet  screen,  no  opportunity  is  given  for  any  considerable 
separation  to  occur  before  the  material  is  compelled  to  leave  the 
jig.     This  means  that  the  percentage  of  refuse   (sink  particles) 
in  the  washed  coal  is  high,  while  the  loss  of  coal  in  the  refuse  is 
also  high.     On  the  other  hand,  if  the  supply  of  raw  coal  should 
be  too  slow,  only  the  best  material  is  carried  away  as  washed  coal. 
The  usual  openings  of  the  refuse  valves  release  a  refuse  which 
necessarily  contains  a  large  percentage  of  good  coal.     The  above 
holds  good  only  if  the  common  type  of  hand  operated  refuse  gates 
are  used.     With  kettle  valves  or  the  * '  Elmore ' '  automatic  refuse 
valves,  no  such  loss  of  good  coal  can  occur. 

(2)  Supply  of  Water.     If  water  is  pumped  into  the  jig  too 
rapidly,  it  is  capable  of  flushing  away  in  the  washed  coal  an 
unduly  large  quantity  of  refuse;  but,  as  only  the  heavier  parts 
of  the  material  can  sink  through  the  rapidly  moving  stream  of 
water  to  the  screen,  the  refuse  contains  little  good  coal, 


JIG  DATA  209 

The  trouble,  which  is  introduced  by  admitting  too  little  water, 
is,  that  at  each  up  stroke  of  the  plunger  there  is  a  strong  suction 
in  the  jig  tank.  This  tends  to  arrange  the  lightest  particles 
near  the  screen.  In  consequence,  the  washed  coal  is  largely  com- 
posed of  refuse,  and  good  coal  forms  a  considerable  portion  of  the 
refuse. 

(3)  Stroke  and  Speed  of  the  Eccentric  Action. —  (a)  Long 
and  Rapid  Stroke.  The  more  violently  the  downward  stroke  of 
the  plunger  is  made,  the  greater  is  the  distance  that  each  particle 
is  separated  from  its  neighbor.  A  better  opportunity  is,  there- 
fore, offered  for  the  materials  to  settle  in  accordance  with  their 
respective  specific  gravities.  A  rapid  upward  pulsation  in  the 
jig  necessitates,  however,  a  rapid  discharge  of  water  from  the 
jig,  and  it  has  been  observed  that  too  strong  a  stream  of  water 
does  not  result  in  effective  separation. 

With  a  long  and  rapid  down  stroke,  it  will  be  essential  for  the 
machinery  to  provide  for  a  somewhat  slower  up  stroke  of  the 
plunger;  otherwise,  the  admission  of  water,  to  prevent  a  suction 
at  the  screen  must  be  large,  and,  in  consequence,  the  ill  effect  of  a 
strong  current  of  water  is  aggravated. 

(b)  Long  and  Moderately  Slow  Stroke.  With  a  long  slow 
stroke,  especially  where  provision  is  made  for  a  slower  up  stroke 
of  the  plunger,  the  best  conditions  are  obtained  for  washing  nut 
coal.  The  long  but  moderately  slow  down  stroke  of  the  plunger 
produces  just  sufficient  agitation  for  the  materials  to  settle  freely 
during  the  slower  up  stroke.  As  water  is  admitted  below  the 
screen,  the  settling  occurs  in  an  almost  still  liquid. 

For  fine  raw  coal  in  a  feldspar-washer  even  a  long  and  mod- 
erately slow  stroke  is  not  essential,  as  the  agitation  is  unneces- 
sarily great,  and  the  rate  at  which  the  coal  is  washed  is  limited. 

Where  fine  coal  is  being  washed  with  larger  sizes,  a  long  and 
somewhat  slow  stroke  is  essential.  Even  then,  the  rapidity  with 
which  the  stroke  must  be  made  to  agitate  the  larger  sizes  causes 
such  a  rush  of  water  from  the  box  that  fine  dirt  is  carried  away 
with  the  coal. 

Hence,  where  unsized  coal  is  washed  and  subsequently  classi- 
fied, the  fine  washed  coal  is  improved,  as  a  rule,  by  a  second 
washing.  Obviously,  if  the  stroke  is  too  slow,  insufficient  free- 
dom is  produced  in  the  washing-box,  and  the  plant  delivers  from 


210  COAL  WASHING 

the  washed  coal  and  refuse  outlets  products  differing  little  from 
the  original  raw  coal. 

(c)  Short  and  Rapid  Stroke.     A  short  stroke,  even  though 
it  is  rapid,  does  not  produce  sufficient  freedom  among  the  parti- 
cles of  nut-coal  to  secure  good  separation.     On  the  other  hand,  a 
short  and  fairly  rapid  stroke  is  admirable  for  the  fine  coal.     The 
short  stroke  brings  about  sufficient  freedom  for  the  particles  to 
settle  easily  during  the  up  stroke  of  the  plunger.     It  is,  of  course, 
essential  that  water  shall  be  admitted  to  prevent  suction  at  the 
wet  screen;  otherwise,  not  only  do  the  materials  settle  in  the  re- 
verse order,  but  a  large  quantity  of  fine  stuff  is  sucked  through 
the  feldspar  bed,  to  be  lost  as  refuse. 

(d)  Short  and  Slow  Stroke.     A  short  and  slow  stroke  would, 
probably,  produce  no  movement  in  a  washing-bed  composed  of 
large  particles,  and  even  with  fine  coal  it  would  be  unsatisfactory, 
as  insufficient  separation  would  be  effected. 

(4)  Refuse  Discharge.  The  refuse  discharge  must  be  so  regu- 
lated that  it  corresponds  with  the  rate  of  accumulation  of  the 
impurities.  If  it  is  too  slow,  the  impurities  pass  out  in  the 
washed  coal,  while  the  refuse  is  confined  to  the  dense  particles  of 
impurities ;  whereas,  if  the  outlet  is  opened  too  frequently,  some 
of  the  coal  escapes  with  the  impurities,  and  only  an  extremely 
high-grade  washed  coal  is  delivered.  A  tabulated  statement  of 
the  above  facts  concerning  jig-washers  is  given  in  Table  33. 

With  such  devices  as  the  trough  and  the  inverted-cone  washers, 
there  is  only  the  possibility  of  varying  the  supply  of  raw  coal 
and  water,  and  of  regulating  the  removal  of  the  refuse.  If  the 
supply  of  raw  coal  or  water  is  too  great,  to  either  the  trough  or 
inverted-cone  washer,  the  washed  coal  contains  a  large  quantity 
of  impurities,  whereas  the  refuse  is  confined  to  the  denser  impuri- 
ties. A  similar  state  of  affairs  is  brought  about  by  releasing  too 
little  refuse.  On  the  contrary,  if  the  supply  of  raw  coal  or 
water  is  too  small,  the  usual  openings  of  the  refuse-valves  dis- 
charge a  refuse  which  contains  a  high  percentage  of  coal.  The 
washed  coal  then  is  of  the  highest  grade.  By  releasing  the 
refuse  too  rapidly  the  same  result  is  produced. 


JIG  DATA 


211 


EFFECT  AND  CAUSE  IN  JIG-WASHING 


Description 

High  percentage  of  coal  in 
refuse 

Low  percentage  of  coal  in 
refuse 

(a)    Feed  of  raw  coal  too 

(a)   Water-supply  too 

rapid. 

great. 

(6)    Strong    "suction"    at 

(6)    Stroke    too    long    and 

wet  screen  due  to  : 

rapid,      with      good 

(i.)  Small  admis- 

water-supply to  pre- 

sion   of    wa- 

vent suction. 

ter. 

(c)    Too   small   a  quantity 

High  percentage 
of  impurities  in 
washed  coal 

(ii.)  L  ong    and 
rapid    stroke 
with    insuffi- 
cient  supply 

of  refuse  released. 

of    water. 

(c)   Stroke  too  slow  (long 

or  short)  . 

(d)   Stroke  too  short  and 

rapid  (  nut-washer  )  . 

Low  percentage  of 
impurities  in 
washed  coal 

(a)    Feed  of  raw  coal  in- 
sufficient. 
(&)    Too  large1  a  quantity 
of  refuse  released. 

All  details  of  washer  suit- 
ably adjusted. 

TABLE  33 


CHAPTER  XXI 
CONSTRUCTION  OF  JIGS 

Jigs  built  of  timber  are  light  in  weight  and  cheap  in  first  cost. 
The  jig  boxes  can  be  repaired  and  even  totally  rebuilt  at  the 
washery.  But  they  are  hard  to  keep  tight  and  not  very  durable. 
The  best  wood  for  the  construction  of  jig  boxes  and  tanks  is  long- 
leaf  yellow  pine  of  heart  specification.  In  some  instances  creo- 
soted  timber  has  been  used.  Timber  construction  is  especially 
advisable  if  the  wash  water  becomes  acidulous. 

Jigs  built  of  steel  plates  are  comparatively  light  but  expensive. 
They  must  be  stiffened  with  angle  irons  and  braces  to  keep  the 
side  plates  from  breathing. 

Cast-iron  jigs  appear  to  have  the  preference  at  present.  They 
are  rather  heavy  but  stiff  and  solid.  They  are  not  affected  by 
acidulated  water  and  always  represent  the  value  of  scrap  iron, 
if  for  any  reason  it  becomes  necessary  to  change  the  type  of  jigs. 

Reinforced  concrete  has  been  used  in  several  instances  in  the 
construction  of  jig  tanks.  This  material,  however,  has  the  great 
disadvantage  that  it  is  affected  by  acidulated  water  to  a  highly 
disastrous  degree.  The  solid,  heavy  construction  of  reinforced- 
concrete  tanks  prevents  also  any  changes  or  alterations.  A  rein- 
forced-concrete  jig  tank  must  remain  the  way  it  has  been  put 
up  in  the  first  place,  or  it  must  be  entirely  rejected  if  changes 
become  necessary. 

The  proper  selection  of  the  material  of  which  jigs  should  be 
built  depends  on  cost  of  construction,  cost  of  upkeep,  nature  of 
the  wash  water  and  the  design  of  the  supporting  structures. 

Side  or  end  plungers  actuated  by  eccentrics  give,  under  normal 
conditions,  perfect  satisfaction.  If  the  coal  is  difficult  to  wash  a 
differential  motion  drive  for  the  plungers  would  be  advisable. 
In  cramped  quarters  considerable  space  can  be  saved  by  the  use 
of  plungers  placed  below  the  screens.  For  extremely  large  jigs 
compressed  air  can  be  used  to  advantage. 

212 


JW  COXKTRLVT10\  213 

The  discharge  of  washed  coal  is  similar  in  all  jigs.  It  consists 
of  a  simple  overflow  dam  or  weir.  The  discharge  of  refuse,  how- 
ever, shows  many  variations  and  has  been  elaborated  previously. 
For  jigs  treating  unsized  or  the  larger  sizes  of  coarse  coal  plain 
gates  can  be  used.  This  type  is  almost  universally  employed  on 
reciprocating  jigs.  The  gates  can  be  either  of  the  slide  or  swing 
type.  The  swing  gates  offer  some  advantages  over  the  slide 
gates  as  they  permit  the  removal  of  the  heaviest  pieces  of  refuse 
without  disturbing  the  bed. 

The  kettle  or  pot  valves  are  to  be  recommended  for  coarse- 
coal  jigs.  They  permit  a  close  regulation  of  the  slate  bed  and 
have  the  further  advantage  that  they  prevent  an  accidental  loss 
of  the  slate  bed,  caused  by  inattention  of  the  operator.  The  ket- 
tle valve  being  open  on  top  permits  an  inspection  of  the  refuse. 
The  double  slate  gate  is  nothing  more  nor  less  than  a  developed 
kettle  valve  extending  over  the  whole  width  of  the  jig.  It  is  at 
present  the  best  design  known,  as  it  combines  the  good  points  of 
the  kettle  valve  with  those  of  a  slide  gate. 

Revolving  slate  valves  are  only  to  be  used  for  fine-coal  jigs,  as 
the  heavy  refuse  accumulating  in  coarse-coal  jigs  is  apt  to  choke 
up  this  type  of  valve.  For  the  removal  of  the  fine  refuse  pass- 
ing through  the  perforations  of  the  screens,  a  common  molasses 
gate  is  the  simplest  and  best  device.  Other  types  of  mechani- 
cally operated  valves  for  the  discharge  of  hutch  work  have 
been  designed,  but  they  all  appear  to  be  entirely  too  compli- 
cated. 

To  facilitate  the  operation  and  supervision  of  the  washing 
process  it  is  desirable  to  locate  all  the  jigs,  including  the  re  wash 
machines,  on  a  common  platform.  This  permits  the  consolida- 
tion of  a  number  of  jigs  in  a  battery  thereby  saving  space  and 
cost  of  installation.  The  rule  is  to  provide  each  compartment  of 
a  jig  with  one  separate  plunger.  If  one  plunger  should  be  used 
for  two  compartments,  there  would  be  great  danger  that  the  jig 
work  of  these  compartments  would  be  uneven  and  unsatisfactory. 
The  second  compartment  is  treating  quite  different  material 
from  the  first,  and  as  explained  previously  length  and  frequency 
of  plunger  stroke  must  be  adapted  to  the  material  to  be  treated. 
So  it  can  be  clearly  seen  that  it  will  be  absolutely  wrong  to  treat 
two  different  materials  with  the  same  kind  of  pulsation,  The 


214 


COAL  WASHING 


jigs  should  be  placed  in  such  position  that  they  will  be  accessible 
from  all  sides ;  and  to  facilitate  the  inspection  of  the  work  per- 
formed they  ought  to  be  placed  somewhat  higher  than  the  operat- 
ing platform. 


CHAPTER  XXIT 
CONCENTRATING  TABLES 

For  the  washing  of  coal  too  fine  to  be  treated  successfully  on 
jigs,  concentrating  tables  are  coming  into  use.  Such  tables  have 
been  employed  for  many  years  in  ore-dressing  plants,  and  the 
principle  applied  for  ore  concentration  can  also  be  used  for  coal 
washing.  The  specific  gravity  of  the  material  and  the  size  of  the 
grains  are  two  of  the  most  important  considerations.  Specific 
gravity  comes  first  and  size  of  grains  is  second  in  importance. 

Many  types  of  concentrators  for  fine  material  have  been  devel- 
oped, but  among  them  the  machines  with  the  separating  surface 
in  motion  and  having  a  continuous  feed  and  discharge  are  the 
most  important.  The  machines  belonging  to  this  group  utilize 
mechanical  agitation  to  separate  the  grains  into  layers.  The 
pulp,  in  its  passage  across  the  table  deck,  stratifies,  in  accordance 
with  the  specific  gravity  of  the  different  particles,  the  heaviest 
seeking  the  lower  strata  between  the  riffles,  while  the  coal  or 
lighter  particles  remain  on  top,  to  be  washed  off  the  side  of  the 
deck  by  the  cross  flow  of  wash  water,  the  heavier,  or  high-ash 
grains,  being  guided  by  the  riffles  and  advanced  to  the  end  of 
the  deck  by  the  head  motion  and  discharged  as  refuse.  This 
separation  is  visible  at  all  times  while  the  table  is  in  operation, 
and  susceptible  of  exact  adjustment  and  under  easy  control  of 
the  operator. 

Concentrating  tables  for  coal  use  riffles  on  the  table  deck. 
These  riffles  are  produced  by  tacking  cleats  on  the  table  surface. 
The  arrangement  of  the  riffles  forms  at  present  the  main  differ- 
ence between  the  different  tables.  Other  differences  can  be 
found  in  the  manner  in  which  the  shaking  motion  of  the  deck  is 
produced.  In  any  case  the  backward  motion  should  be  faster 
than  the  forward  motion.  The  movement  is  either  parallel  to  the 
direction  of  the  riffles  or  the  riffles  are  placed  at  an  angle  to  the 
direction  of  the  table  motion.  In  some  cases  this  angularity  of 

215 


216 


COAL  WASHING 


the  riffles  is  only  carried  on  for  a  short  distance  in  the  middle  of 
the  deck.  Whatever  the  direction  of  the  riffles,  the  table  motion 
moves  the  material  toward  the  refuse  side  of  the  machine.  The 
refuse  collected  on  the  bottom  of  the  riffles  is  carried  forward 
faster  than  the  clean  coal  forming  the  upper  strata  of  the  bed. 
The  wash  water,  however,  washes  the  clean  coal  down  the  slope 
of  the  table  much  faster  than  the  deeper  stratified  refuse. 

There  are  now  employed  for  coal  washing  five  distinct  types  of 
concentrating  tables,  all  of  them  adopted  from  the  same  types 
used  in  ore  dressing. 


Fig.  122.    Massco  Coal  Table 


The  Massco  Coal  Washing  Table,  illustrated  in  Fig.  122  is, 
strictly  speaking,  a  Wilfley  table  used  for  coal  washing. 

The  Massco  table  consists  essentially  of  a  linoleum-covered 
riffled  deck  about  7  ft.  wide  by  16  ft.  long,  transversely  inclined 
and  reciprocated  endwise  by  a  head  motion  mechanism.  The 
crushed  coal,  previously  mixed  with  about  three  times  its  quan- 
tity of  water,  is  fed  onto  this  deck  through  a  feed-box  at  the 
upper  corner.  The  power  required  for  each  table  is  about 
%  h.p.,  and  the  capacity  with  %  in.  coal  has  been  found  to  be 
from  six  to  eight  tons  per  hour.  The  table  should  run  at  about 
220  r.p.m.  The  deck  ought  to  have  a  side  inclination  of  about 
3  in.  from  the  feed  to  the  coal  discharge  side.  Soon  after  the 


CONCEN TRA  77 A  (/   TA  BLE8  217 

coal  lias  been  fed  onto  the  table  a  line  of  separation  between  the 
washed  coal  and  waste  will  become  apparent  in  a  generally  di- 
agonal direction  from  the  feed-box  toward  the  waste  discharge 
corner.  By  varying  the  inclination  of  the  deck  the  location  of 
this  line  will  change.  The  nearer  horizontal  the  deck,  the  higher 
up  this  line  will  terminate,  and  vice  versa ;  but  the  proper  loca- 
tion of  this  line  is  such  that  it  shall  terminate  at  the  lower  waste 
discharge  corner. 

There  is  a  combination  of  elements  that  bring  about  this  result, 
viz. :  the  length  of  stroke  imparted  by  the  head  motion ;  the  side 
inclination  controlled  by  the  hand-wheel;  the  quantity  of  wash 
water,  and  the  end  elevation  of  table.  The  latter  is  accomplished 
by  the  aid  of  the  adjusting-screws  in  the  bottom  bar  for  the 
slipper-bearing,  and  may  vary  from  one-half  to  one  inch  in  the 
length  of  table;  normally  higher  at  waste-discharge  end  than 
head-motion  end  of  table.  Under  proper  working  conditions  the 
visible  waste  on  the  deck  should  occupy  the  area  represented  by 
a  triangle,  one  side  of  which  is  the  waste-discharge  end  of  the 
table,  and  whose  base  is  a  line  drawn  from  the  lower  discharge 
corner  to  the  center  of  the  wash-water  box,  and  the  space  between 
the  riffles  within  this  triangle  should  be  kept  completely  filled 
with  waste  material.  Should  the  waste  material  advance  too  fast 
to  allow  these  spaces  to  remain  filled  with  advancing  waste  it 
indicates  that  the  stroke  is  too  long  or  the  end  inclination  insuffi- 
cient, and  should  be  adjusted  to  fill  the  above  conditions. 

The  receiving  launder  on  the  coal-discharge  side  for  the 
cleaned  coal  should  be  provided  with  a  movable  diverting-spout, 
to  carry  such  material  to  waste  as  is  not  suitable  to  mix  with 
cleaned  coal. 

The  Butchart  table,  shown  in  Fig.  123,  is  designed  and  built 
to  supply  the  demand  for  a  better  machine,  both  mechanically 
and  metallurgically,  than  has  been  available  heretofore — a  thor- 
oughly well-built  table,  capable  of  treating  heavy  as  well  as  light 
tonnages,  and  withstanding  the  continuously  hard  service  to 
which  such  apparatus  is  subjected.  It  is  the  product  of  long  ex- 
perience in  the  operation  and  construction  of  concentrating 
tables,  and  has  been  greatly  improved  since  its  introduction  upon 
the  market  five  years  ago. 

Simple  design,  rugged  construction,  mechanically  correct  deck 


218  COAL  WASHING 

suspension,  permanently  efficient  tilting  mechanism,  self-con- 
tained and  fully  enclosed  drive,  steel  base,  automatic  lubrication 
of  every  bearing,  accessibility  of  all  parts,  elimination  of  small 
pieces,  great  capacity  and  high  efficiency,  and  its  adaptability  to 
all  kinds  of  feed,  mark  it  as  a  leader  in  this  class  of  apparatus. 
The  base  of  the  table  consists  of  two  heavy  steel  channels, 
bolted  to  either  cast  iron  or  wood  sub-sills,  which  receive  the 
foundation  bolts.  The  channels  are  bent  inwardly  at  the  drive 
mechanism  end,  bringing  their  combined  strength  toward  the 
longitudinal  center  and  under  the  point  of  greatest  stress.  The 
cast  iron  sub-sills  have  lugs  which  bolt  to  the  web  of  the  channels 


Fig.  123.     Butchart  Table 

instead  of  through  the  flanges.  The  reciprotor  is  bolted  direct 
to  the  flanges  of  the  base  channels. 

On  the  base  channels  are  bolted  two  cast  iron  double  pedestals 
providing  suspension  bearings  for  the  tilting  beams,  which  carry 
the  deck.  The  tilting  beams  are  heavy  castings  which  permit 
suspension  of  the  deck  below  its  center  of  gravity.  Their  ends 
are  formed  into  receptacles  for  the  sliding  bearings.  The  swing 
of  the  deck  is  always  on  the  center  line  of  pull  rod  and  suspen- 
sion bearings. 

Dimensions  of  the  deck,  15  ft.  6  in.  by  6  ft.  0  in.  Side  sills  and 
lengthwise  stringers  are  of  carefully  selected  lumber  bolted  or 
riveted  to  steel  plates.  The  rear  end  and  side  are  reinforced 
with  a  continuous  6  in.  steel  plate,  protected  from  contact  with 
water  by  the  manner  in  which  the  linoleum  cover  is  attached. 


CONCENTRATING  TABLES  219 

The  other  two  sides  are  reinforced  with  a  continuous  steel  plate 
protected  by  cover  boards,  so  that  no  iron  work  is  in  direct  con- 
tact with  feed  or  wash  water.  The  reinforced  longitudinal 
stringers  are  bolted  to  malleable  iron  cross  trusses  which  support 
the  deck  upon  its  sliding  bearings  and  will  not  corrode.  The  con- 
nection to  the  pull  rod  of  reciprotor  or  drive  mechanism  is-  a 
strong,  easily  removable  malleable  iron  casting  bolted  to  two  of 
the'  longitudinal*  stringers,  insuring  proper  distribution  of  the 
driving  stresses.  The  deck  is  carried  on  four  sliding  bearings, 
the  entire  construction  being  on  the  principle  of  the  cantilever 
steel  bridge,  as  distinguished  from  that  of  the  wooden  'trestle. 
The  method  of  construction  and  suspension  prevent  sagging, 
war-ping  or  bulging  of  the  deck  surface.  Upon  loosening  one 
nut  the  deck  may  be  lifted  off  the  mounting. 

The  mechanism  for  tilting  the  deck  is  one  of  the  most  impor- 
tant parts  of  the  concentrating  table  and  has  been  given  special 
attention.  It  consists  of  two  pairs  of  right  and  left  hand  bronze 
screws  of  large  diameter  working  in  internally  threaded  and  pro- 
tected sleeves  actuated  by  a  hand-wheel,  shaft  and  6  in.  bevel 
gears.  One  turn  of  the  hand-wheel  gives  1  in.  change  in  slope  of 
deck.  The  tilting  screws  are  directly  attached  at  their  upper 
and  lower  ends,  respectively,  to  the  tilting  beams  and  to  brackets 
bolted  to  base  channels  and  sub-sills.  Wear  is  automatically  com- 
pensated and  can  cause  no  lost  motion  in  gears,  both  ends  of  the 
deck  are  moved  simultaneously  and  positively,  automatically 
retained  at  the  desired  slope,  and  twisting  of  the  deck  is  impos- 
sible. Any  movement  of  the  hand-wheel,  however  slight,  pro- 
duces a  corresponding  change  in  the  slope  of  the  deck.  Quick, 
close  and  positive  adjustment  is  afforded,  all  parts  are  strongly 
constructed  and  there  are  no  set-screws,  cams,  wedges,  turn- 
buckles  or  other  small  parts  to  wear  out,  work  loose,  rust,  require 
adjustment  or  replacement. 

The  reciprotor  or  drive  mechanism  is  of  a  modified  toggle  type, 
compact,  self-contained  and  completely  enclosed  by  a  cast  iron 
housing  and  cover  which  absolutely  prevent  entrance  of  dust, 
water  and  sand.  There  are  no  screw  adjustments  except  for 
spring  tension  and  bearing  caps  on  drive  shaft,  no  set-screws, 
bolts  or  ot'her  parts  to  work  loose  or  require  attention.  The 
drive  shaft  bearings  are  supported  on  the  base  channels,  are 


220  COAL  WASHING 

automatically  oiled  and  fitted  with  removable  babbit  liners  which 
can  be  replaced  in  a  few  moments,  no  pouring  of  babbit  being 
required  at  any  time.  The  drive  is  at  the  center  of  gravity  of 
the  deck  and  through  the  spring,  eliminating  all  vibration  and 
unnecessary  strains  on  bearings. 

The  housing  serves  as  a  container  for  a  supply  of  oil  sufficient 
for  several  months'  running.  Lubrication  of  every  bearing  is 
accomplished  by  oil  spraying  discs  carried  by  the  drive  shaft. 
Automatic  lubrication  results  in  cool  running  bearings  and  great 
economy  in  oil  consumption,  about  one  pint  being  required  at 
intervals  of  three  or  four  months.  Oiling  requires  no  attention 
other  than  renewal  of  the  supply  at  long  intervals.  The  mecha- 
nism is  practically  noiseless  in  operation  and  nearly  fool-proof ; 
there  are  no  bearings  to  rebabbit  and  nothing  for  the  repair  man 
to  tinker  with.  No  attention  or  adjustment  is  required  until 
working  parts  are  worn  out.  Repair  parts  are  so  simple  in  de- 
sign and  construction  that  they  are  furnished  at  a  few  cents  per 
pound. 

The  differential  action  is  strong  and  proportionate  for  all 
lengths  of  stroke,  the  range  being  from  l/>  to  1H  in.  The  stroke 
adjustment  is  positive  and  permanent,  is  made  without  the  use 
of  screws  or  additional  parts,  and  cannot  be  tampered  with  by 
the  operator  without  stopping  the  table.  This  is  one  of  the  most 
important  features  of  the  reciprotor,  as  it  prevents  the  loss  of 
concentrate  frequently  caused  on  some  tables  by  the  operator 
shortening  the  stroke  to  stop  "pounding"  of  the  driving  mecha- 
nism or  noise  in  the  bearings.  Very  slight  spring  tension  is  re- 
quired and  less  power  is  consequently  consumed,  this  being  about 
7Ao  h.p.  regardless  of  load. 

The  table,  as  regularly  furnished,  is  driven  from  line  shaft  in 
the  usual  manner,  but  can  also  be  fitted  with  an  individual  1  h.p. 
motor  mounted  on  the  base  channels  driving  by  means  of  a  short 
belt.  The  individual  belted  motor  offers  many  advantages,  such 
as  decreased  power  consumption  due  to  absence  of  long  belts  and 
line  shafts,  convenience  in  starting  and  stopping,  and  freedom  to 
place  tables  wherever  desired  regardless  of  location  of  line  shafts. 
Dangerous  and  unsightly  shafting,  pullej^s  and  belts  are  done 
away  with,  leaving  the  entire  overhead  space  open  for  launders, 
pipes,  light,  etc.  About  16  ft.  of  2Vz  in.  3-ply  rubber  belt  are 


COXCEXTltATING  TABLES  221 

required  instead  of  40  or  more  feet  of  4  in.  4-ply.  The  motor  is 
protected  from  dust,  water  and  sand,  takes  up  no  floor  space,  is 
readily  accessible  and  belt  may  be  tightened  without  stopping 
the  table.  The  installation  of  individual  motors  costs  no  more 
than  a  single  motor  with  line  shaft,  hangers,  tight  and  loose  pul- 
leys, and  necessary  belting. 

As  all  oiling  is  automatic  and  practically  no  mechanical  ad- 
justments are  required,  the  operator  can  give  his  entire  atten- 
tion to  regulation  of  feed,  dressing  water  and  the  production  of 
concentrates  of  the  desired  grade.  Little  alteration  in  the  slope 
of  the  deck  is  required,  insuring  a  high  average  efficiency  even 
with  inexperienced  labor.  Under  ordinary  conditions,  about  all 
that  is  necessary  is  to  adjust  the  table  to  the  load  and  then  let  it 
alone. 

The  ultimate  value  of  any  concentrating  table  consists  in  its 
ability  to  separate  and  save  the  good  coal.  However  excellent  its 
mechanical  design  and  construction,  the  table  is  useless  unless 
concentration  be  successfully  accomplished,  and  its  relative 
worth  is,  therefore,  largely  dependent  upon  the  concentrating 
surface  employed.  The  causes  of  the  greatly  increased  capacity 
and  efficiency  of  this  table  are  found  primarily  in  the  riffling 
system  and  secondarily  in  its  mechanical  features. 

On  many  classes  of  feed  the  table  will  handle  from  two  to  five 
times  as  heavy  loads  as  can  be  treated  on  similar  machines,  being 
governed  principally  by  the  rapidity  with  which  any  particular 
class  of  feed  will  stratify,  this  action  depending  upon  the  char- 
acteristics of  the  material.  If  feed  is  properly  prepared  the 
values  are  mechanically  carried  into  concentrate  without  loss  of 
any  portion  into  middlings  or  tailings.  The  means  by  which 
these  results  are  obtained  may  be  summarized  as  follows: 

The  entire  deck  surface  is  riffled  and  caused  to  perform  useful  work  other 
than  mere  distribution  of  dressing  water. 

A  few  riffles  near  the  higher  side  of  the  deck  segregate  and  discharge  a 
large  percentage  of  the  readily  separable  refuse  usually  present  in  table 
feed,  the  remainder  of  the  surface  being  free  to  handle  the  less  easily  re- 
coverable refuse,  make  a  two  part  separations,  clean  middlings,  etc. 

The  channel  between  each  two  riffles  is  a  complete  concentrating  surface, 
receiving  its  load,  stratifying,  cleaning  and  discharging  its  quota  of  finished 
refuse:  the  raw  coal  does  not  drift  diagonally  across  the  deck  and  conse- 
quently there  is  po  true  refuse  in  the  middle  product, 


222  COAL  WASHING 

The  refuse  is  cleaned  between  relatively  deep  riffles,  permitting  the 
handling  of  much  heavier  loads  of  raw  coal  than  those  systems  in  which 
cleaning  is  done  between  shallow  riffles  or  upon  an  unriffled  surface. 

Classification  is  accomplished  upon  the  table  itself. 

The  riffles  are  strips  of  pine  or  other  suitable  wood,  cut  out  with 
a  circular  saw,  require  little  hand  work  and  can  be  made  cheaply. 
To  facilitate  bending,  they  are  soaked  before  laying,  and  prac- 
tically make  their  own  curves  as  they  are  nailed  in  place.  Two 
men  familiar  with  the  work  can  apply  a  set  in  about  3  hours. 

The  deck  surface  is  in  one  plane,  completely  riffled  and  divided 
into  three  distinct  zones  by  deflections  in  the  riffles  themselves, 
the  purpose  of  these  areas  being  stratification,  cleaning  and  dis- 
charge of  concentrate. 

The  deepest  portions  of  the  riffles,  lying  directly  in  front  of 
the  feed  distributor  and  extending  to  the  line  marking  the  com- 
mencement of  the  curves,  form  a  series  of  deep  grooves  in  which 
stratification  is  accomplished,  the  refuse  accumulating  at  the  bot- 
toms of  the  channels  and  being  carried  forward  by  the  differ- 
ential action  of  the  driving  mechanism. 

The  cleaning  zone  consists  of  an  area  in  which  the  riffles  are 
bent  toward  the  higher  side  of  the  table,  the  deflection,  in  com- 
bination with  the  transverse  inclination  of  the  deck  surface,  pro- 
ducing channels  sloping  toward  the  rear  or  feed  end  of  the  table 
and  in  which  currents  of  water  will  flow  in  a  direction  the  reverse 
of  that  in  which  the  material  is  moved  by  the  differential  action 
of  the  driving  mechanism. 

The  discharge  area  is  supplied  with  riffles  of  sufficient  depth 
to  prevent  drifting  of  refuse  across  the  deck  and  to  cause  its 
rapid  discharge  over  the  end.  When  it  is  necessary  to  employ 
deep  riffles  in  the  discharging  zone,  a  secondary  deflection  or 
terminal  upward  curve  is  used  to  prevent  too  great  a  propor- 
tion of  the  dressing  water  being  carried  over  the  end  of  the 
deck.  This  arrangement  insures  a  uniform  flow  of  dressing 
water  over  the  entire  deck  surface,  so  that  the  cleaning  action  of 
the  upper  riffles  is  duplicated  and  supplemented  by  those  nearer 
the  discharge  side. 

The  operation  of  the  riffle  system  is  as  follows: 

The  table  being  started,  dressing  water  is  supplied,  successively 
overflows  from  one  to  another  of  the  riffles,  and  upon  reaching 


CONCENTRATING  TABLES  223 

the  cleaning  zone  follows  the  downwardly  sloping  channels  to- 
ward the  rear  or  feed  end  of  the  table.  The  quantity  of  dressing 
water  required  is  from  150  to  300  gals,  per  ton  of  feed,  depend- 
ing upon  the  fineness  to  which  it  is  ground. 

When  feed  reaches  the  upper  riffles,  stratification  quickly 
ensues  and  the  greater  portion  of  the  refuse  is  deposited  in  the 
first  15  channels,  the  remaining  refuse  being  caught  further  down 
the  table.  The  differential  action  of  the  reciprotor  moves  the 
entire  mass  forward  until  it  reaches  the  cleaning  zone.  Here  it 
meets  the  streams  of  dressing  water  flowing  in  the  opposite  direc- 
tion and  the  superficial  layer  of  good  coal  is  washed  away,  while 
the  refuse,  by  reason  of  its  greater  specific  gravity,  continues  its 
forward  movement.  As  the  riffles  in  the  cleaning  zone  are  not 
parallel  to  the  line  of  table  motion,  they  produce  a  transverse 
agitation  or  "side-shake,"  similar  in  effect  to  that  of  the  vanner, 
causing-  any  remaining  good  coal  to  be  brought  to  the  surface, 
whence  it  is  washed  back  into  the  main  body  of  middlings  or 
tailings.  When  the  refuse  reaches  the  further  side  of  the  clean- 
ing zone,  it  has  been  freed  from  good  coal  and  passes  into  the 
discharge  zone  of  straight  riffles,  which  carry  it  to  the  end  of 
the  deck. 

The  following  table  gives  the  results  obtained  with  a  Butchart 
table  treating  Illinois  coal,  which  contains  at  least  2  per  cent,  of 
organic  sulphur.  These  results  were  not  obtained  during  a  test 
run,  but  in  the  course  of  actual  washing  operation. 

RESULTS  OF  A  BUTCHART  TABLE  TREATING  %>  ix.  UNSIZED  ILLINOIS  COAL 


Raw 
Ash 
per  cent. 

Coal 
Sulphur 
per  cent. 

Washed 
Ash 
per  cent. 

Coal 
Sulphur 
per  cent. 

Refuse 
Ash                  Sulphur 
per  cent.           per  cent. 

13.72 

3.11 

6.11 

2.20 

49.10 

0.38 

13.80 

3.  1C 

6.37 

2.21 

57.27 

10.16 

13.07 

3.30 

6.60 

2.24 

51.75 

10.18 

13.10 

3.20 

6.00 

2.13 

45.30 

8.14 

13.70 

3.16 

6.15 

2.16 

50.10 

0.03 

13.20 

3.23 

6.40 

2.27 

40.20 

10.04 

13.G5 

3.42 

6.31 

2.21 

55.15 

8.78 

13.34 

3.CO 

7.14 

2^32 

54.50 

0.75 

12.64 

3.05 

6.36 

2.16 

51.60 

10.33 

14.30 

3.20 

5.90 

2.20 

45.80 

8.50 

Average 

13.45 

3.20 

6.33 

2  22 

50.07 

0.52 

TABLE  34 


224  COAL  WASHING 

The  Deister-Overstrom  Table  is  shown  in  Fig.  124. 

The  primary  features  of  the  construction  of  the  Deister-Over- 
strom table  are  the  diagonal  deck  and  the  "'pool"  riffling.  By  a 
combination  of  these  in  the  treatment  of  coal,  highest  efficiency 
is  secured  in  extraction  of  good  coal  with  a  clean  refuse  and  but 
little  middle  product.  High  extraction  and  clean  refuse  are  se- 
cured: (1)  By  the  diagonal  disposition,  in  the  line  of  travel 
taken  by  the  coal,  of  the  principal  concentrating  section  of  the 
deck,  which  insures  the  coal  being  retained  thereon  for  the 


Fig.  124.     Deister-Overstrom  Table 

longest  possible  time;  (2)  all  riffles  are  placed  parallel  to  the 
line  of  motion  and  at  an  angle  to  the  coal  discharge.  This  se- 
cures the  advantage  of  deflected  riffles  without  whipping  action, 
and  gives  to  the  stratified  material  a  free  and  unobstructed  move- 
ment towards  the  refuse  edge  of  the  table.  Clean  refuse  is  pro- 
duced by  the  spreading  out  in  a  thin  or  shallow  sheet  of  the 
refuse  bed,  thus  exposing  all  of  the  fine  coal  to  the  action  of  the 
dressing  water. 

A  small  proportion  of  middle  product  is  maintained  by  reason 
of  the  non-congestion  of  the  refuse  at  the  middling  corner.  The 
spreading  out  of  the  refuse,  which  thus  prevents  banking  against 
the  good  coal,  enables  all  of  the  free  refuse  to  come  forward 
without  interruption. 

The  coal  traversing  the  deck  surface  takes  an  oblique  course, 
which  is  the  resultant,  mainly,  of  the  action  on  its  particles  of 
two  forces,  the  gravital  action  due  to  the  side  tip  of  the  deck,  as- 
sisted by  the  side  wash  of  the  feed  and  the  dressing  water,  and 
the  forward  and  lengthwise  impulse  imparted  through  the  deck 


CONCENTRATING  TABLED  225 

surface  by  the  differential  or  "kick"  of  the  motion  assisted  by 
the  riffles.  Assuming  the  same  tip  in  all  cases,  the  deeper  the 
riffling,  the  deeper  and  narrower  will  be  the  pulp  zone  as  it 
traverses  the  deck,  and  the  less  its  angle  of  obliquity. 

Shallow  riffling  has  just  the  opposite  effect.  It  permits  the 
coal  to  spread  out  in  a  broad,  thin  zone.  In  concentrating  on  a 
long  and  narrow  rectangular  deck,  the  riffling  must  be  relatively 
deep  to  confine  the  operation  to  the  working  area  of  the  deck  and 
deliver  the  coal  and  the  refuse  at  their  proper  places.  On  such 
a  deck,  therefore,  the  pulp  occupies  a  comparatively  narrow 
zone,  leaving  large  unoccupied  areas  which  contribute  but  slightly 
to  the  effectiveness  of  the  operation. 

The  disposition  of  the  deck  area  of  the  Deister-Overstrom  table 
is  such  as  to  take  the  greatest  possible  advantage  of  the  natural 
obliquity  of  the  pulp  flow.  Shallower  riffles  can,  therefore,  be 
used  on  the  Deister-Overstrom  deck  (the  maximum  height  on 
Deister-Overstrom  sand  tables  is  only  %2  in.)  and  the  pulp 
spreads  out  in  a  thin  zone.  This  permits  a  freer  inter-movement 
of  the  particles,  a  better  stratification  and,  therefore,  a  better 
concentration  and  separation  of  the  good  coal  from  the  refuse. 

Head  Motions.  The  Deister  heavy-duty  head  motion  has  been 
in  successful  use  for  the  last  ten  years.  Its  design  embodies  the 
combination  of  two  differential  motions,  which  are  adjustable  in 
varying  relation,  to  produce  a  gentle  or  a  very  sharp  differential 
or  "kick"  in  the  movement  transmitted  to  the  deck.  Both  the 
length  of  stroke  and  degree  of  differential  are  adjustable. 
Changes  can  be  made  in  either  while  the  table  is  in  operation. 

The  stroke  is  regulated  by  turning  a  hand  piece.  Turning  to 
the  right  will  lengthen  it.  Fig.  125  gives  an  X-Ray  picture  of 
this  head  motion. 

The  variation  of  differential  is  secured  by  shifting  the  lug  of 
the  adjustable  eccentric  14  (see  Fig.  125).  Usually  the  best  re- 
sults are  obtained  when  this  lug  (measuring  from  the  rear  inside 
edge  of  its  cup)  is  Vs  in.  forward  (toward  the  table)  v from  a  verti- 
cal through  the  center  of  the  shaft. 

Shifting  the  lug  toward  the  table  will  have  a  tendency  to 
hasten  the  advance  of  the  settled  mineral  toward  the  concentrate 
edge,  while  the  reverse  has  a  retarding  effect.  In  general  the  lug 
position  for  a  proper  differential  will  lie  somewhere  between  the 


226 


COAL  WASHING 


vertical  and  a  forward  shift  of  45  deg.  In  shifting  the  adjust- 
able eccentric  loosen  the  nuts  of  the  cap  holding  the  bearing  of 
which  it  is  an  extension.  After  the  adjustment  is  made  be  sure 
that  the  nuts  are  tight.  It  is  advisable  not  to  move  the  lug  for- 
ward or  backward  more  than  a/4  in.  at  a  time,  as  a  small  adjust- 
ment will  make  a  very  perceptible  difference  in  the  movement  of 
the  pulp.  The  motion  should  run  forward  or  towards  the  table. 
This  motion  is  equipped  with  an  8  in.  roller  eccentric  and  16  in. 
driving  pulley. 


Fig.  125.     Deister  Heavy-Duty  Head  Motion 

The  Overstrom  Head  Motion  is  strong,  compact,  exceedingly 
simple  and  has  few  wearing  parts.  A  variation  in  stroke  from 
9ie  in.  to  ^16  in.  can  be  secured  by  the  adjusting  screw.  The 
central  position  of  the  block  and  screw  gives  a  %  in.  stroke.  This 
head  motion  should  be  run  forward  or  toward  the  table.  Fig. 
126  shows  the  construction  details  of  the  Overstrom  Head  Motion. 

The  Overstrom  Motion  and  the  Deister  Heavy -Duty  Motion 
are  interchangeable  in  their  use  on  the  Deister-Overstrom  di- 
agonal deck  tables.  Connection  between  the  head  motion  and  the 
rocker  arm  is  by  means  of  a  connecting  link  and  steel  yoke. 

OPERATION  DATA  ON  THE  DEISTER-OVERSTROM   CONCENTRATING  TABLE 


Recommended  speed  in  r.p.m 

Length  of  stroke 

Driving  pulley   dimensions 


240  to  265 
•14  in.  to  1  in. 
16  in.  by  4  in. 


CONCENTRATING  TABLES 


227 


Recommended  belt  dimensions 3  in.,  3-ply  belt 

Maximum    power    required !*/£  h.p. 

Recommended  percentage  of  solids  in  feed 35  per  cent. 

Figs.  127  and  128  show  a  typical  arrangement  of  a  table  coal 
washing  plant  designed  by  the  Deister  Concentrator  Company. 

The  Deister  Plat-0  Coal  Washing  Table  is  shown  in  Fig.  129. 
The  distinctive  feature  of  this  table  is  an  elevated  plateau  with 
inclined  approaches.  The  table  requires  from  %  to  1  h.p.  and 


Fig.  126.     Overstrom  Head  Motion 

occupies  a  floor  space  of  7  ft.  by  17  ft.  4^  in.  From  3  to  5  gal. 
of  dressing  water  are  required  per  minute.  The  capacities  of  the 
table  are  as  follows : 

Through  %   in.   screen  from 8-10  tons 

Through  %   in.   screen  from 10-12  tons 

Through  ¥2  in.   screen  from 11-13  tons 

Through  %  in.   screen  from 12-14  tons 

The  Overstrom  Universal  Table  has  some  features  in  which  it 
differs  from  all  other  devices  of  its  kind. 

The  head-motion  consists  of  an  unbalanced  pulley  driving  loose 


228 


COAL  WASHING 


- 


Ol 
c't 


CONCENTRATING  TABLES 


229 


on  a  shaft  rigidly  attached  to  the  table  deck,  this  motion  being 
limited  by  a  fixed  stop  on  one  end  of  the  stroke  and  a  cushion 
spring  at  the  other,  thus  doing  away  with  all  eccentrics,  cams 
and  toggles. 

There  are  no  bearings  under  the  deck,  but  it  is  supported  from 
the  floor  frame  by  laminated  wooden  springs  which  allow  the 
table  to  swing  lengthways  as  an  inverted  pendulum,  the  motion 
being  in  the  arc  of  a  circle,  the  riffles  also  being  laid  out  in  arcs 
practically  parallel  to  the  line  of  motion. 

The  supporting  legs  are  inclined  slightly  backward  toward  the 


Cross  Section  Through  Coal  Washery  Using  Tables 


head-motion,  causing  the  table  to  rise  on  its  forward  stroke.  On 
account  of  the  method  of  imparting  reciprocating  motion  to  the 
table,  it  will  automatically  increase  or  decrease  the  stroke  with 
a  heavier  or  lighter  feed. 

Fig.  130  illustrates  the  Overstrom  universal  concentrating 
table.  On  this  table  the  motion  of  the  deck  is  practically  parallel 
to  the  direction  of  the  riffles.  The  advantages  claimed  for  this 
table  are  as  follows :  As  the  deck  is  tilted,  the  height  of  the  riffle 
tips  above  their  feed  ends  increases.  The  refuse  must  climb  the 
height  corresponding  to  the  curvature  of  the  riffles  and  the  tilt 


230  COAL  WASHING 

of  the  deck.  The  grade  increases  as  the  riffle  tips  are  ap- 
proached, causing  more  and  more  of  the  good  coal  to  fall  back 
and  pass  over  the  coal  discharge  side.  The  separation  of  coal 
and  refuse  is  gradual  and  takes  place  over  a  large  surface.  It  is 
not  confined  to  one  congested  line  or  zone.  The  motion  of  the 
deck,  parallel  to  the  riffles  throughout  their  length,  causes  the 
refuse  to  travel  with  the  riffles,  not  against  an  arbitrary  curve, 


Fig.  129.     Deister  Plat-0  Table 

either  in  riffles  or  deck.     Therefore  the  travel  is  rapid  and  th< 
capacity  is  great. 

J.  B.  Morrow,  in  his  paper  on  ' i  Coal  Washing  on  Concenti 
ing  Tables, ' '  has  the  following  to  say : x 

The  chief  virtue  of  the  table  as  a  coal  cleaner  lies  in  its  sensitiveness  to 
adjustment  and  full  visibility  of  the  process,  together  with  the  ease  with 
which  the  quality  of  the  product  may  be  varied.  The  variables  as  they 
affect  practice  are  as  follows:  (1)  Length  of  stroke,  (2)  revolutions  per 
minute,  (3)  lateral  inclination,  (4)  longitudinal  inclination,  (5)  dimen- 
sion and  spacing  of  riffles. 

As  an  illustration  of  what  can  be  accomplished  on  a  machine  of  this  kind, 

i  "Coal  Washing  on  Concentrating  Tables,"  by  J.  B.  Morrow.  Coal  Age 
(Vol.  16,  No.  13,  1919). 


CONCENTRATING  TABLES 


231 


the  figures  that  follow  show  the  result  of  a  test  run  made  on  an  Overstrom- 
Universal  table. 

The  feed  to  the  table,  which  consisted  of  jig  hutch  and  reground  middlings 
ranging  in  size  from  %  in.  down  to  fine  sludge,  is  a  harder  proposition  to 
handle  than  the  primary  coal  on  account  of  the  concentration  of  the  bony 
matter,  some  of  which  will  only  have  a  small  differential  in  specific  gravity 
to  distinguish  it  from  the  rock  or  coal. 


Fig.  130.     Overstrom  Universal  Table 

The  tonnage  handled  was  five  tons  per  hour,  having  a  composition,  as 
shown  by  the  specific  gravity  separation,  of 

46  per  cent,  feed  at  48.0  per  cent,  ash 
54  per  cent,  feed  at  11.3  per  cent,  ash 

100  per  cent,  feed       28.2  per  cent,  ash 
The  results  obtained  were: 

49  per  cent,  clean  coal  with,  11.6  per  cent,  ash 
40.6  per  cent,  clean  rock  with  48.7  per  cent,  ash 
10.4  per  cent,  middlings  with  27.5  per  cent,  ash 

100      per  cent.  27.66  per  cent,  ash 

This  is  equivalent  to  a  recovery  of  91  per  cent,  of  the  coal.     If  the  mid- 
dlings were  put  in  with  the  waste,  it  would  give  51  per  cent,  waste  with 
an  ash  of  44.1  per  cent,  as  compared  with  48.7  per  cent,  in  the  theoretical 
separation,  carrying  10  per  cent,  of  recoverable  coal. 
In  practice,  these  middlings  from  th§  primary  machines  are  again  treated 


232 


COAL  WASHING 


on  a  table  using  a  shorter  stroke  and  lower  riffles  and  the  final  waste  from 
the  mill  contains  an  average  of  5  per  cent,  of  recoverable  coal,  or,  expressed 
in  another  way,  99  per  cent,  of  the  recoverable  coal  in  the  feed  is  reclaimed. 

In  ore  dressing  the  line  between  concentrates  and  tailings  can 
be  usually  clearly  defined,  but  with  coal  this  is  not  possible.  Be- 
tween clean  coal  and  pure  refuse  we  find  a  more  or  less  wide  area 
containing  the  so-called  middle  products.  Therefore  it  becomes 
necessary  to  make  either  three  products  or  to  establish  the  divi- 
sion between  coal  and  refuse  at  the  most  economical  and  efficient 
point.  Fig.  131  shows  clearly  the  segregation  of  the  particles 


I 


Weight  of  coal  run  over  table  -8965  Ib. 


19 
376 

7 

IS 
1980 
8  0 

17    1   16       15   1   14       13   1   12  1    11 
2852  2836  6000  6784  7446  7492  3464 
7  6|    6    1    7    1    9    JIO.SJ   II       II 
No.  1  Coal  ->|<-N< 

.0  1   9    1    8 
2342  1906  2456 
11  2|  13      15 

).  ?Coal~4<-No 

7 

2824 
23 
.  2Rc 

6 
1962 
30 

ck-» 

Number 

A'cight  in  gran 
>cr  ccut  ash 

Equal*  33  per  cent  of  total  -4750  ib.          1  1  J%  .  1017  Ib.  10.7%  -957  Ib. 

9.0  per  cent  ash                    1  1  J  percent  ash  22  per  cent  ash 

~SS      -* 


Ml 

£    I  • 
\    x 


Mixture  of  No.  1  and  2  coal  .64.3  per  cent  of  tout  weight  -9.5  per  cenfaih 

Mature  of  No.  1  and  2  coal  and  No.  2  rock  -  75  per  cent  of  total  weight  - 1 L3  per  cent  ash 

Fig.  131 

according  to  their  specific  gravity,  which  is  influenced  by  the 
amount  of  ash. 

The  materials  contained  in  raw  coal  do  not  show  an  increase 
in  specific  gravity  by  clearly  pronounced  steps.  The  specific 
gravities  increase  by  infinitesimal  increments,  and  if  we  plat  the 
specific  gravities  of  the  different  materials  we  will  get  a  con- 
tinuous ascending  curve  and  not  a  broken  straight  line  inter- 
rupted by  distinct  steps. 

The  proper  washing  of  fine  coal  has  for  a  long  time  been  an 
uncertain  undertaking.  With  the  advent  of  concentrating  tables, 
however,  the  difficult  problem  of  washing  fine  coal  has  been  solved 
in  a  satisfactory  manner.  With  coking  coal  there  now  exists  the 


TABLES  233 

possibility  of  abandoning  the  use  of  jigs  entirely,  by  crushing 
all  the  coal  to  the  required  fineness  and  treating  it  on  tables. 
This  fine  crushing  will  liberate  more  effectively  the  bone  coal, 
and  tables  will  therefore  produce  cleaner  washed  products.  The 
reason  for  the  foregoing  is  obvious.  The  finer  the  coal  is 
crushed  the  more  complete  is  the  physical  separation  of  coal  from 
its  impurities,  and  it  only  remains  now  to  accomplish  the  segre- 
gation of  the  coal  from  them. 

Tables  require  little  power,  about  1  h.p.  is  sufficient  to  operate 
one  table.  The  supporting  structure  for  a  table  can  be  much 
lighter  than  for  jigs,  and  the  building  housing  such  machines 
does  not  need  to  be  so  massive. 

Besides  primary  washing,  tables  can  be  used  to  good  advantage 
either  for  the  rewashing  of  recrushed  middle  products  or  for  the 
re  washing  of  the  sludge  recovered  from  the  water  clarification. 
For  this  latter  purpose  a  slightly  different  type  of  machine 
should  be  used,  resembling  the  slime  tables  used  in  ore  dressing. 
Concentrating  tables  can  be  used  economically  at  every  washery, 
and  at  present  they  represent  the  most  feasible  apparatus  for 
treating  fine  coal  and  sludge.  They  unquestionably  offer  many 
advantages  in  an  economical  as  well  as  purely  technical  way. 

The  cost  of  installation  is  low,  they  require  less  expensive 
buildings  and  foundations  than  jigs.  The  cost  of  operation  is 
considerably  less  than  that  of  jigs  on  account  of  the  small  power 
required,  the  reduced  wear  and  tear  and  less  supervision.  One 
operator  can  watch  five  times  as  many  tables  as  jigs.  In  regard 
to  capacity  tables  are  equal  to  jigs  treating  the  same  kind  of  coal. 
Outputs  up  to  12  tons  per  hour  have  been  recorded  for  one  table. 
Fig.  131  indicates  clearly  the  manner  in  which  the  separation 
is  effected  on  one  of  these  machines  and  shows  the  possibility  of 
making  any  kind  of  washed  product  or  refuse  by  simply  dividing 
the  outflowing  product  at  any  desired  point  along  the  side  or  end 
of  the  table. 

The  great  advantage  of  a  table  lies  in  the  fact  that  the  process 
of  separation  is  immediately  before  the  eye  and  any  desired 
change  can  be  made  without  interrupting  the  operation.  The 
outflow  of  the  materials  is  visible,  and  by  a  judicious  regulation 
of  the  stroke,  the  water  supply  and  the  slope  of  the  table,  the 
operator  lias  full  control  over  the  process. 


234 


COAL  WASHING 


The    following   tables   show    some    results   obtained    in   table 
work: 


Weight,    Lb.         Per   Cent. 


Ash, 
Per   Cent. 


Sulphur, 
Per   Cent. 


Raw    coal    481  100  12.50  2.67 

Washed  coal   406  84.4  6.71  2.04 

First  middle  product...  30.5  6.3  29.19  5.72 

Second  middle  product.  .  38.5  8.0  42.65  6.02 

Refuse    6.0  1.3  65.00  10.54 

— — — — ___ 

SEPARATION  BY  SPECIFIC  GRAVITY 

Lighter   than    1.37    sp.    gr.        Heavier  than   1.37   sp.   gr. 
Washed  coal 82.2  per  cent.  17.8  per  cent. 

Lighter  than  Between    1.35  Heavier  than 

1.35    sp.   gr.  and  1.50  sp.  gr.  1.50   sp.   gr. 

First  middle  product ...      12.5  per  cent.         21.0  per  cent.         66.5  per  cent. 
Second  middle  product.      11.4  per  cent.         12.7  per  cent.         75.9  per  cent. 

Lighter   than    1.45    sp.  -gr.      Heavier   than   1.45   sp.   gr. 
Refuse    , . 2.2  per  cent.  97.8  per  cent. 

RESULTS  OBTAINED  ON  TABLES  WITH  SULPHUR  ELIMINATION 


Raw 
Coal 


Washed  Side  End 

Coal          Middlings     Middlings 


Refuse 


Elimina- 
tion in 
per  cent. 


Per  cent, 
sulphur 


1.47 
1.25 
1.37 
1.30 


1.50 
1.38 
1.55 
1.37 


3.02 
2.33 
2.43 
2.55 


24.30  55.7 

23.28  63.9 

14.68  59.9 

16.94  58.3 


Raw   coal    sul- 

p  h u r ,  per 

cent 2.76  2.93  2.76  3.21  3.59  3.75  3.08  3.53  4.06  4.74 

Washed  coal 

sulphur,   per 

cent  1.26  1.38  1.29  1.48  1.43  1.88  1.59  1.37  1.37  1.34 

Elimination  in 

per  cent.    ..    54.3     52.9     53.3     53.9     60.2     49.9     48.4     61.2     66.2     71.7 


CHAPTER  XXIII 
FURTHER  TREATMENT  OF  THE  PRODUCTS  OF  A  JIG 

The  products  delivered  from  the  jig  cannot  be  used  without 
further  treatment.  All  the  products  require  a  more  or  less  ex- 
tensive and  difficult  subsequent  preparation  before  they  can  be 
placed  on  the  market.  This  after-treatment  depends  largely 
upon  the  size  of  the  coal.  Coarse  coal  must  be  dewatered,  classi- 
fied and  delivered  into  the  loading  or  storage  bins,  avoiding 
thereby  breakage  and  abrasion  as  much  as  possible. 

Fine  coal  must  be  dewatered  and  delivered  into  the  bins  or 
railroad  cars.  Middle  products  are  either  first  crushed  and  then 
delivered  to  rewash  jigs  or  are  delivered  to  these  machines  with- 
out previous  crushing.  The  refuse  from  the  rewash  jigs  is  com- 
bined with  that  from  the  primary  jigs,  and  the  washed  coal  is 
either  mixed  with  that  from  the  primary  jigs  or  separately  stored 
in  bins  as  boiler-house  fuel,  according  to  its  degree  of  purity. 
The  wash  water  must  be  clarified  before  it  can  be  put  back  into 
circulation. 

The  sludge  recovered  by  the  process  of  water  clarification  can 
be  subjected  to  a  further  treatment,  but  in  any  case  it  must  be 
dewatered  before  it  can  be  mixed  with  the  washed  products.  If 
the  sludge  is  of  such  a  character,  however,  that  subsequent  treat- 
ment will  not  improve  it,  it  must  be  wasted  and  thrown  on  the 
refuse  dump. 

The  refuse  is  delivered  into  refuse  storage  bins  and  from  there 
by  means  of  self -dumping  cars  or  similar  devices  carried  to  the 
dump.  Before  delivery  into  the  bin,  however,  the  refuse  must 
be  dewatered  by  means  of  a  perforated  bucket  elevator ;  the  over- 
flowing dirty  water  is  clarified  and  put  back  into  circulation. 
The  resulting  sludge  is  deposited  on  the  refuse  dump.  If  the 
refuse  contains  pyrites,  a  separate  installation  for  its  recovery 
would  be  advisable.  The  economic  value  of  such  a  plant,  how- 
ever, would  depend  a  great  deal  upon  the  market  value  of  sul- 
phur. 

235 


CHAPTER  XXIV 
SUBSEQUENT  TREATMENT  OF  WASHED  NUT  COAL 

The  nut  coal  as  it  leaves  the  jigs  has  been  either  subjected  to 
a  preliminary  sizing  or  it  has  been  washed  unsized.  Even  if  the 
raw  coal  has.  been  sized  before  washing,  a  subsequent  sizing  be- 
comes necessary  on  account  of  the  degradation  suffered  by  it 
in  the  jigs.  Therefore  the  different  sizes  of  washed  coal  are  not 
kept  separate  after  washing,  but  all  the  coal  coming  from  the 
coarse  coal  jigs  is  conveyed  together  to  the  final  sizing  screens. 
This  conveying  is  best  accomplished  by  means  of  the  wash  water 
in  sluice-ways.  Usually  all  the  washed  coal  is  sluiced  into  a 
settling  tank,  out  of  which  a  dewatering  elevator  lifts  it  to 
the  sizing  screens,  which  are  located  on  top  of  the  loading  bins. 
To  secure  a  still  better  draining  off  of  the  water,  a  dewatering 
screen  is  placed  ahead  of  the  sizing  screens. 

Dewatering.  For  dewatering  of  nut  coal  either  fixed  or 
movable  screens  are  used.  Fixed  screens  have  the  disadvantage 
of  taking  up  too  much  height  on  account  of  the  necessarily  steep 
pitch  at  which  they  must  be  placed  to  let  the  coal  slide  down. 
The  coal  is  also  subjected  to  a  considerable  drop,  which  causes 
some  degradation. 

Shaking  screens  or  revolving  screens  are  to  be  preferred.  Fig. 
61  (page  119)  shows  a  dewatering  and  resizing  plant.  This  ar- 
rangement has  only  single  screens  and  the  coal  is  sized  from  fine 
to  coarse.  A  resizing  screening  plant  from  coarse  to  fine  is 
shown  in  Fig.  132. 

This  screening  plant  consists  of  two  separate  screens  "a"  and 
"b."  Both  screens  are  driven  from  one  shaft  "c,"  having  two 
cranks  placed  at  180  deg.,  which  arrangement  balances  the  two 
screens. 

In  most  of  the  Stewart  type  washeries  for  fuel  coal  the  washed 
material  is  sized  in  revolving  screens,  which  are  located  over 

236 


TREATMKXT  <)/•'   WASHED  NUT  COAL 


237 


a  long  and  narrow  sludge  recovery  tank.  The  different  sizes 
of  coal  are  conveyed  by  spouts  to  the  foot  of  separate  ele- 
vators which  carry  the  sized  coal  to  the  respective  loading  bins. 
The  flow  sheet  on  page  362  and  the  general  plan  of  a  fuel  coal 
washery  on  page  369  showrs  such  an  arrangement.  Two  revolving 
screens  are  used  in  tandem.  The  first  screen  has  three  sections 
with  different  perforations.  In  the  first  section  all  the  No.  4 
and  No.  5  coal  together  with  the  wash-water  are  screened  out. 
In  the  second  section  No.  3  coal  is  separated  and  the  undersize 
of  the  third  section  is  No.  2  coal,  while  the  oversize  is  No.  1  coal. 
The  No.  4  and  No.  5  coal  and  all  the  wash-water  are  sluiced  into 
the  second  screen  which  makes  No.  5  as  the  undersize  and  No.  4 


Fig.  132.     Resizing  Screens  on  Top  of  Washed  Coal  Bins 

as  the  oversize.  The  No.  5  drops  in  the  sludge  tank  and  is  de- 
watered  and  carried  out  by  a  dewatering  elevator,  which  dis- 
charges the  coal  011  a  shaking  screen  with  very  fine  perforations, 
where  several  sprays  of  fresh  water  wash  off  the  adhering  fire- 
clay and  the  muddy  water. 

The  following  tables  taken  from  Bulletin  No.  69  of  the  Engi- 
neering Experiment  Station  of  the  University  of  Illinois,  en- 
titled "Coal  Washing  in  Illinois,"  by  F.  C.  Lincoln,  give  the 
sizes  and  capacities  of  revolving  screens  used  for  resizing  washed 
coal  which  had  been  sized  prior  to  washing. 

Table  36  gives  data  of  revolving  screens  used  for  sizing  washed 
coal  which  had  not  been  sized  before  washing. 


238 


COAL  WASHING 


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TREATMENT  OF  WASHED  NUT  COAL 


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CHAPTER  XXV 
THE  STORAGE  OF  WASHED  NUT  COAL 

It  is  desirable  to  load  the  washed  coal  into  the  railroad  cars 
as  quickly  as  possible  so  as  to  pass  a  continuous  string  of  cars 
through  the  washery  and  to  avoid  delays  in  switching.  This 
can  not  be  accomplished  by  loading  directly  from  the  jigs.  It  is 
necessary  to  accumulate  the  washed  coal  in  large  quantities  in 
bins.  For  each  size  of  coal  a  separate  bin  or  compartment  must 
be  provided  and  in  some  washeries  a  mixing  conveyor  is  installed 
on  top  of  the  bins  to  permit  the  mixing  of  two  or  more  sizes. 

The  location  of  the  loading  bins  is  determined  by  the  loading 
tracks,  since  the  loading  chutes  may  be  either  directly  over  the 
cars  or  to  one  side  of  the  track. 

The  size  of  the  bins  depends  upon  the  capacity  of  the  washer 
and  the  amount  of  each  size  made.  It  is,  however,  considered 
better  to  make  all  the  compartments  of  the  same  size  and  large 
enough  to  hold  two  carloads  each  or  about  100  tons.  This  per- 
mits loading  of  railroad  cars  without  waiting  for  more  coal  to 
come  from  the  jigs  to  make  a  full  carload. 

The  washed  coal  bins  on  account  of  their  great  capacity  have  a 
considerable  height  and  it  is  not  permissible  to  let  the  coal  drop 
direct  from  the  screens,  without  breaking  the  fall.  Several 
methods  are  in  use  to  prevent  this  injurious  drop.  Simple 
chutes  are  shown  in  Fig.  132.  Such  chutes  are  sufficient  for 
hard  coal,  but  for  the  more  friable  varieties  spiral  chutes  as 
shown  in  Fig.  61,  or  so-called  telegraph  chutes  are  used.  Spiral 
chutes  are  of  a  simple  construction  and  comparatively  cheap, 
but  the  pitch  of  the  spiral  must  be  steep  in  order  to  allow  the 
coal  to  slide  easily.  Telegraph  chutes  are  mostly  built  of  wood 
and  lined  with  steel  plates.  They  are  arranged  on  the  inside  of 
the  bin  walls  and  let  the  coal  slide  down  without  perceptible  drop. 

To  prevent  the  freezing  of  the  washed  coal  in  the  winter  time 
steam  heated  discharge  gates  are  used  and  the  room  under  the 
bins  is  housed  in  and  steam  heated. 

240 


CHAPTER  XXVI 
THE  CRUSHING  OF  COAL 

Coal  as  it  comes  from  the  mine  is  crushed  for  three  purposes : 

1.  The  crushing  of  the  lump  coal  into  smaller  sizes,  suitable 
for  washing,  results  at  the  same  time  in  the  breaking  off  of  the 
adhering  slate  and  pyrite. 

2.  The  crushing  of  the  middle  products,  to  free  the  good  coal 
from  bone  and  slate. 

3.  The  crushing  of  the  washed  nut  coal  to  make  it  suitable  for 
coking. 

The  Crushing  of  Lump  Coal.  The  size  of  a  crushing  plant  de- 
pends upon  the  percentage  of  lump  in  the  run  of  mine  coal  and 
on  the  demand  for  lump  coal.  The  condition  in  the  mine  and 
the  demand  for  lump  coal  are  not  always  of  such  a  uniformity 
that  either  a  crushing  plant  would  not  be  required  or  that  such 
a  plant  could  be  operated  with  a  constant  capacity.  Unforeseen 
changes  in  the  operation  of  a  mine,  such  as  a  shut-down  of  a  cer- 
tain section  of  the  mine  or  changes  in  the  characteristics  of  the 
coal,  etc.,  or  sudden  heavy  demands  for  certain  sizes  result  at 
times  in  great  changes  in  the  amount  of  crushing  a  plant  has  to 
handle. 

Sometimes  a  crushing  plant  must  handle  a  large  tonnage  of 
lump  coal  in  a  short  time  in  order  to  fill  orders  for  a  certain  size, 
whereas  at  other  times  the  plant  will  be  idle.  Consequently  a 
crushing  plant  should  be  designed  for  large  capacities.  Crushers 
require  considerable  power.  As  a  result,  if  crushers  are  driven 
by  motors,  which  drive  at  the  same  time  other  steady-running 
machinery,  the  motors  must  be  much  bigger  than  are  required 
for  the  steady-running  machinery,  if  the  crushing  plant  is  idle. 
This  means  an  inefficient  power  consumption  and  for  this  reason 
a  crushing  plant  ought  to  be  driven  by  independent  motors. 

Careful  Crushing.  The  crusher  that  produces  the  largest  per- 
centage of  the  desired  size  with  the  least  amount  of  over  and 

241 


242  COAL  WASHING 

undersize,  especially  dust,  is  the  best.  The  fact  that  the  efficient 
washing  of  coal  increases  in  difficulty  as  the  size  of  the  material 
diminishes  demands  that  the  production  of  fine  raw  coal  should 
be  restricted  to  a  minimum.  For  fuel,  the  raw  coal  is  usually 
crushed  to  pass  a  3  in.  ring,  but  for  coking  coal  the  most  efficient 
size  has  not  been  fully  determined  and  depends  to  a  great  degree 
upon  the  characteristics  of  the  raw  coal.  It  varies  between 
%  and  11A  in.  T.  J.  Drakeley  shows  in  his  ''Scientific  Study  on 
Coal  Washing"  that  the  maximum  reduction  in  ash  content  is 
obtained  when  washing  material  of  a  diameter  of  about  1U  in. 
and  he  says  that  any  reduction  in  the  diameter  beyond  this  limit 
results  in  a  rapid  decrease  in  the  efficiency  of  the  separation. 
For  a  washery  to  be  used  advantageously  the  material  sent  to  it 
must  consist  of  a  merely  mechanical  admixture  of  impurities  with 
coal.  A  washery  obviously  fails  entirely  in  dealing  with  coal 
that  is  "intergrown"  with  impurities.  Such  raw  coal  needs 
judicious  crushing  so  that  the  impurities  are  freed  from  their 
attachment  to  the  coal  without  undue  production  of  fines.  Wash- 
ing then  may  be  an  effective  means  of  purification.  Crushing 
can  not  be  indulged  in  to  an  unlimited  extent.  Jungst  con- 
cluded from  his  investigations  that  the  limit  of  fineness  according 
to  quality  is  %o  to  #20  in.  Experience  with  American  washeries 
show  that  coal  finer  than  Vw  in.  can  not  be  dealt  with  profitably. 

There  appears  to  be  no  known  process  for  separating  the  slate 
dust  from  the  finest  raw  coal  dust.  Although  in  such  cases, 
where  the  whole  of  the  washed  coal  is  sent  to  the  coke  ovens,  there 
is  no  waste,  it  should  be  borne  in  mind  that  the  fine  washed  coal 
is  always  of  an  inferior  quality  and  that  a  coal  which  has  been 
washed  more  successfully  in  a  larger  size  might  yield  a  better 
coke.  Drakeley  has  compiled  a  table  showing  the  average  con- 
centration and  the  ash  contents  of  different  sizes  up  to  2  in.  in 
diameter. 

From  this  table  it  is  to  be  concluded  that  in  washing  the  larger 
sizes  a  higher  concentration  of  the  valuable  constituent  is  ef- 
fected. Therefore,  every  effort  should  be  made  to  limit  the 
breakage,  so  as  to  preserve  the  large  pieces  of  material.  Any 
preventable  reduction  of  the  diameter  of  the  particles  of  raw 
coal  to  less  than  %  in.  involves  a  considerable  lowering  of  the 
attainable  quality  of  the  washed  product. 


THE  CRUSHING  OF  COAL 


243 


CONCENTRATION  OF  FLOAT  PARTICLES   IN  AND  ASH  CONTENT  OF  THE 
VARIOUS  SIZED  WASHED  COAL 


Size 

Average 
Diameter 

Average 
Concentration 
of  the 
Float  Particles 

Average 
Ash  Content 

Inches 

Inches 

per  cent. 

per  cent. 

0-% 

VlG 

00.14 

10.34 

%-%& 

%2 

90.75 

9.84 

%<*-% 

%2 

91.80 

9.15 

%-%6 

%2 

92.19 

9.03 

%6-% 

»)fa 

92.57 

8.81 

%-% 

%6 

93.90 

8.06 

"  1/2-11/16 

!%2 

94.41 

8.04 

1J/16-% 

2%2 

94.41 

8.01 

%-l 

7/6 

94.70 

7.92 

1-1  % 

IMa 

95.07 

7.79 

l1^-!^ 

1% 

95.14 

7.73 

!%-!% 

1%* 

95.14 

7.76 

!%-!% 

1% 

95.15 

7.82 

194-2 

1%. 

95.15 

7.85 

Average  raw  coal  . 

81.21 

16.82 

TABLE  37 


Types  of  Crushers.     For  the  crushing  of  coal,  needle  crushers, 
double  roll  crushers,  single  roll  crushers,  hammer  crushers  and 


a 


Fig.  133.     Needle  Crusher  for  Coal 


244 


COAL  WASHING 


Bradford  breakers  are  used.  Needle  crushers  are  used  largely  in 
Europe  but  thus  far  have  not  found  great  favor  with  the  Ameri- 
can coal  operator.  Figs.  133  and  134  show  such  a  crusher. 

The  swinging  plate  "  a "  has  rigidly  fastened  to  it  steel  needles 
"b"  which  decrease  in  length  and  diameter  from  top  to  bottom. 
The  coal  drops  on  the  plate  "c"  under  the  biggest  and  longest 
needles  which  crack  the  largest  lumps.  The  coal  then  drops  on 
the  shaker  screen  "d"  which  derives  its  motion  from  the  shaft 


Fig.  134.     Needle  Crusher 


of  the  swinging  plate  by  means  of  a  lever  "e."  This  screen 
cushioned  on  its  rear  end  by  a  spring  "f. "  The  perforations  of 
the  screen  determine  the  size  of  the  coal.  On  the  screen  the  coal 
is  subjected  to  the  action  of  the  smaller  needles. 

The  only  type  of  needle  crushers  built  in  America  is  shown  in 
Fig.  135.  A  comparative  test  made  with  this  crusher  against  a 
roll  crusher  gave  the  following  results: 

When  mine  run  was  crushed  with  a  roll  crusher,  about  30  per 
cent,  of  the  shipments  from  the  washery  were  No.  1  Nut  (over 
1"^  in.)  while  shipments  from  the  washery  where  the  needle 


I 


THE  CRUSHING  OF  COAL 


245 


crusher  was  used  were  about  45  per  cent,  of  No.  1  Nut ;  an  in- 
crease of  15  per  cent,  in  the  quantity  of  the  highest  priced  prod- 


Fig.  135.     Sauerman  Needle  Crusher 


uct.     Of  this  15  per  cent.,  10  per  cent,  came  from  the  decrease  in 
No.  4  and  No.  5  and  the  remaining  5  per  cent,  came  from  the 


Fig.  136.     Toothed  Roll  Crusher 

decrease  in  quantity  of  No.  2  and  No.  3.     The  tests  were  made 
under  the  same  conditions  and  with  coal  from  the  same  bed. 
Roll  crushers  can  be  divided  in  cracking  rolls  that  take  the 


246  COAL  WASHING 

largest  lumps  coming  from  the  mine  and  crush  them  to  ab( 
3  in.,  and  crushing  or  finishing  rolls  which  reduce  the  3  in.  co* 
to  any  desired  size,  usually  %  in. 

Fig.  136  shows  the  photograph  of  a  pair  of  cracking  rolls  with 


Fig.  137.     Crusher  with  Rolls  for  Fine  Crushing 

manganese  steel  teeth  cast  solid  with  the  shell.     Fig.  137  s 

a  pair  of  finishing  rolls.     In  both  of  the  above  illustrations  the 

housing  over  the  rolls  has  been  removed. 

Fig.  138  shows  a  complete  set  of  crushing  rolls  with  housing 


Fig.  138.     Crushing  Rolls  with  Housing  Over  Rolls  and  Gears 

in  place  as  well  as  over  the  gear  wheels,  as  built  by  the  Vulcan 
Iron  Works. 

The  following  table  taken  from  Coal  Age   (Vol.  15,  No.  14, 
1919)  gives  results  of  tests  made  with  roll  crushers: 


777 /•;  CRUSHING  OF  COAL 


247 


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248  COAL  WASH  IXC 

Rolls  and  hammer  mills  are  the  two  distinct  types  1  of  machines 
extensively  used  for  preparing  coal  for  coking.  Since  it  is  pos- 
sible to  purchase  either  type  of  machine  in  almost  any  size  and 
with  the  assurance  that  the  design  and  construction  are  adequate 
for  the  work  intended,  the  choice  of  type  can  be  made  strictly  on 
the  basis  of  suitability  and  economy. 

There  are  certain  advantages  and  disadvantages  that  are  in- 
herent in  each  type  of  machine,  and  these  are  generally  well 
recognized.  Of  greater  importance,  and  less  generally  appre- 
ciated, are  the  characteristics  of  each  machine  for  a  particular 
size  and  service. 

Hammer  mills  pound  and  force  the  coal  through  perforations 
or  longitudinally  placed  bars,  the  size  and  spacing  determining 
the  size  of  the  largest  pieces  of  the  coal.  Rolls  crack  the  coal  by 
compression  and  no  interfering  impediment  obstructs  its  free 
passage  from  the  machine  after  passing  between  the  rolls,  the  dis- 
tance apart  the  rolls  are  set  determining  the  size  of  the  largest 
pieces  of  the  coal.  The  difference  in  principle  of  reduction  evi- 
dently results  in  a  considerable  saving  of  power,  in  favor  of 
rolls,  that  is  worthy  of  consideration  in  making  a  choice  of  types 
on  the  basis  of  economy  of  operation  and  repair.  Cracking  the 
coal  by  compression  in  place  of  pounding  and  forcing  it  through 
a  certain  size  of  hole,  results  in  a  more  uniform  and  better  prod- 
uct for  the  effective  operation  of  the  succeeding  cleaning  method. 
The  pounding  and  forcing  action  creates  an  excessive  amount  of 
fines  and  dust  from  both  the  impurities  and  the  coal,  which  in- 
creases the  difficulties  involved  in  any  cleaning  method  on  a 
commercial  scale  or  of  separating  the  worthless  matter  from  the 
valuable  coal.  It  also  adds  to  the  difficulties  of  clarifying  the 
wash  water  of  refuse. 

Since  rolls  act  on  the  principle  of  cracking  by  compression  and 
since,  when  they  are  set  to  crack  the  coal  to  any  particular  size, 
the  particles  smaller  than  that  size  can  tumble  through  without 
being  further  reduced,  rolls  yield  a  smaller  percentage  of  fines 
and  dust  and  a  more  uniform  finished  product  for  the  final 
cleaning  or  charging  than  any  other  type  of  coal-reduction  ma- 
chine. Rolls  will  crack  wet  coal  as  satisfactorily  as  when  the 

i  "Rolls  for  the  Preparation  of  Coking  Coals."  Coal  Age  (Vol.  15,  No. 
14,  1919). 


THE  CRUSHING  OF  COAL  249 

coal  is  dry  without  increase  in  the  absorption  of  power  and  with- 
out choking.  Approximately  25  per  cent,  less  power  is  required 
with  rolls  than  with  any  other  type  of  reduction  machine  to  re- 
duce like  coal  to  the  same  degree  of  fineness.  The  upkeep  and 
depreciation  per  ton  of  coal  handled  is  barely  noticeable  owing 
to  the  few  parts  subjected  to  wear  and  tear.  In  case  of  choking, 
rolls  can  be  relieved  promptly  without  derangement  of  the  cover- 
ing. They  are  practically  dustless  and  operate  at  a  compara- 
tively slow  speed,  have  a  large  capacity  per  square  foot  of  work- 
ing surface  and  a  minimum  of  skill  and  attendance  is  required. 
Instant  and  rigid  adjustments  may  be  secured  in  the  space  be- 
tween the  rolls ;  automatic  adjustments  for  instantly  relieving  the 
rolls  in  case  tough  foreign  matter  is  mixed  with  the  feed  may  be 
employed  as  well  as  magnetic  attachment  for  the  removal  from 
the  feed  of  nails,  nuts,  mining-machine  cutters  and  the  like.  A 
positive  automatic  feeding  device  to  prevent  overloading  may  be 
employed. 

Rolls  are  comparatively  reasonable  in  price  and  cost  of  installa- 
tion. The  relation  between  the  maximum  size  of  coal  fed  to 
rolls,  the  speed  at  which  they  revolve,  the  diameter  and  the 
space  between  them  determine  the  angle  of  nip.  With  a  spacing 
of  Me  in.  in  the  clear,  and  a  peripheral  speed  of  about  1,500  ft. 
per  minute,  the  relation  between  the  diameter  of  rolls  and  maxi- 
mum size  of  individual  pieces  of  coal  is  as  follows : 

Diameter  of  rolls  24  in.  for  %  in.  cubical  form  of  lumps  and  less 
Diameter  of  rolls  30  in.  for  l*/4  in.  cubical  form  of  lumps  and  less 
Diameter  of  rolls  36  in.  for  1%  in.  cubical  form  of  lumps  and  less 
Diameter  of  rolls  42  in.  for  2*/4  in.  cubical  form  of  lumps  and  less 
Diameter  of  rolls  48  in.  for  3  in.  cubical  form  of  lumps  and  less 

TABLE  39 

In  discussing  this  question  of  crushing  under  different  condi- 
tions there  are  four  factors  to  be  considered:  (a)  Compressive 
strength  of  material;  (b)  extent  of  crushing  desirable;  (c)  work 
or  power  required  for  crushing;  (d)  comparison  of  various  ma- 
chines. 

There  are  four  ways  that  force  may  act  in  crushing  coal  or 
other  material:  (1)  By  direct  pressure  as  between  rolls  where 
there  is  a  strong  force  acting  at  low  velocity;  (2)  by  a  blow  on 
an  anvil,  as  in  stamps,  where  there  is  a  medium  force  acting  at  a 


250 


COAL  WASHING 


moderate  velocity;  (3)  by  a  blow  in  space,  as  in  the  hammer 
mill  or  Carr  disintegrator,  where  there  is  a  weak.force  acting  at 
high  velocity;  (4)  by  grinding,  as  in  the  amalgamating  pan.  In 
the  first  three  cases  the  force  acts  perpendicularly  to  the  surface 
to  produce  rupture  by  compression;  in  the  last  case  it  acts 
obliquely,  producing  rupture  by  compression  combined  with 
shearing. 


Fig.  139 


Crushing  rolls  act  upon  the  lump  C,  Fig.  139,  on  the  principle 
of  the  toggle  joint.  The  revolving  rolls  being  held  in  position 
in  their  journals,  act  radially  upon  the  lump,  gradually  drawing 
it  toward  the  narrowest  space  between  them  and  finally  breaking 
it  by  virtue  of  a  compressive  force  superior  to  the  breaking 
strength  of  the  lump.  The  lump  is  therefore  broken  by  compres- 


Fig.  140 


sion.  The  spaces  between  rolls  vary  from  rolls  close  together  up 
to  ¥2  in.  apart.  The  relation  between  the  diameter  of  lump  fed 
to  rolls  and  the  space  between  them,  that  is  to  say  the  amount  of 
reduction,  is  highly  important  if  rolls  are  to  do  their  best  work. 
If  the  rolls  CD  (Fig.  140)  be  fed  with  a  lump  of  coal  E,  the 
tangent  to  the  rolls  at  AA,  the  point  of  contact  with  the  lump, 
meet  below,  forming  an  angle  2N,  the  half  of  which  N  is  called 


THE  CRUSHING  OF  COAL  251 

the  angle  of  nip.  This  angle  may  have  values  from  0  deg.,  where 
the  space  between  rolls  is  as  large  as  the  feed  lump,  increasing 
until  the  angle  is  so  large  that  the  rolls  cannot  nip  the  frag- 
ments. This  angle  of  nip  in  any  case  will  depend  for  its  value 
upon  the  diameter  of  the  rolls,  the  diameter  of  the  lump  fed  and 
the  distance  in  the  clear  at  which  the  rolls  are  set.  It  is  affected 
by  the  following  factors:  It  is  diminished  by  increasing  the 
diameter  of  the  rolls,  by  increasing  the  space  (clear  distance) 
between  the  rolls,  and  by  diminishing  the  size  of  the  lumps  fed 
to  the  rolls. 

A  comparison  of  Figs.   141  and  143  shows  that  large  rolls, 
acting  on  a  given  size  of  lump,  have  smaller  angles  of  nip  than 


Fig.  141  Fig.  142 


Fig.   143  Fig.  144 

have  small  rolls.  Figs.  141  and  142  show  that  larger  spaces 
give  smaller  angles  of  nip.  Figs.  141  and  144  show  that  smaller 
lumps  give  smaller  angles  of  nip. 

There  are  two  values  of  this  angle  of  nip  which  are  of  special 
interest ;  namely,  when  its  value  equals  the  angle  of  friction, 
and  the  rolls  do  no  work ;  and  when  its  value  becomes  the  practi- 
cal angle  of  nip,  at  which  rolls  will  work  satisfactorily.  The 
angle  N,  Fig.  145,  becomes  the  angle  of  friction  when  it  is  of 
such  a  value  that  a  sphere  fed  to  the  rolls  will  just  slip  upon  the 
points  of  contact  and  therefore  fail  to  be  crushed. 

All  relations  between  size  of  feed,  space  between  rolls,  radius 
of  rolls  and  angle  of  nip  can  be  expressed  by  a  simple  formula, 
which  is  derived  as  follows  (see  Fig.  145):  If  b  =  radius  of 
sphere  to  be  crushed,  a  =  ¥2  space  between  rolls,  N  =  angle  of 
nip  and  r  =  radius  of  roll  =  ¥2  diameter,  then 


252  COAL  WASHING 

r  +  a 

=  cosine  N 


Theoretically,  increase  of  speed,  provided  the  reduction  in 
size  is  sufficiently  slight,  can  be  made  to  almost  any  limit;  but 
practically,  high  speed  with  any  considerable  reduction  will  give 
trouble,  owing  to  the  refusal  of  the  rolls  to  nip  or  take  hold  of  the 
lumps.  These  fly  back  until  a  dangerous  amount  collects  and 
then  the  rolls  choke.  This  may  be  explained  as  follows:  A 
lump  of  coal  falling  under  the  influence  of  gravity  from  heights 
of  6,  12,  18  and  24  in.  will  have  final  velocities  of  340,  481,  589 


Fig.  145 

and  681  ft.  per  minute  respectively.  Now,  if  the  rolls  are  re- 
volving at  900  ft.  per  minute  peripheral  speed,  then  a  certain 
part  of  the  friction  must  be  used  to  accelerate  the  lump  of  coal 
to  this  speed  before  it  will  be  nipped.  This  amount  will  be 
greater  or  less  according  as  the  peripheral  speed  of  the  roll  ex- 
ceeds the  velocity  of  the  particle  by  much  or  little.  The  use  of 
a  part  of  the  friction  for  the  purpose  of  accelerating  the  particle 
does  not  in  itself  prevent  the  particle  from  being  finally  nipped, 
but  merely  delays  the  nipping  action.  It  is  this  delay  during  the 
time  necessary  for  accelerating  the  particle  which  prevents  the 
nipping,  for  until  accelerated  to  the  speed  of  the  rolls  the  parti- 
cle is  necessarily  slipping  and  this  slipping  smooths  the  surface 
to  a  certain  extent  which  causes  the  coefficient  of  friction  to  be 
reduced  and  prevents  the  particle  from  going  through. 

Rolls  of  large  diameter  apparently  possess  three  advantages 
over  those  of  small  diameter:  (1)  The  increased  surface  allows 
more  feed  to  be  crushed  with  a  single  pair  of  rolls,  but  the  gain 
is  not  important  unless  the  renewals  in  the  case  of  the  smaller 


THE  CRUSHING  OF  COAL  253 

rolls  are  so  frequent  as  to  cause  serious  delay  and  added  cost. 
The  wear  of  rolls  per  ton  crushed  would  probably  be  the  same  in 
both  cases.  (2)  The  larger  rolls  can  make  a  greater  reduction  in 
size  of  lump,  the  angle  of  nip  and  the  peripheral  speed  being  the 
same  in  both  cases.  (3)  Larger  rolls  have  a  greater  capacity 
than  smaller  ones,  the  reduction  being  the  same,  since  they  can 
be  run  at  a  higher  rate  of  speed  on  account  of  their  more  ad- 
vantageous angle  of  nip.  In  case  both  the  reduction  and  pe- 
ripheral speed  are  the  same  for  the  large  and  small  rolls,  the 
large  rolls  will  make  the  reduction  more  gradually  and  hence 
with  less  shock. 

Some  authorities  advocate  running  one  of  the  rolls  slightly 
faster  than  the  other  in  order  to  prevent  the  exact  mating  of 
the  rolls  with  a  consequent  possible  unevenness  of  wear  resulting 
therefrom.  This  is  especially  true  with  geared  rolls.  The  use 
of  any  considerable  differentiation  of  this  kind  to  produce  grind- 
ing, with  a  view  of  increasing  the  crushing  power,  has  been 
proved  fallacious  on  hard  brittle  materials,  since  such  an  action 
requires  increased  power  without  corresponding  benefit.  In  re- 
gard to  soft  and  friable  material,  however,  the  case  is  different. 
The  material  which  is  soft,  when  crushed  by  smooth  rolls  running 
at  equal  speeds  forms  ribbons  or  pancakes,  while  a  differential 
adjustment  tears  the  material  apart,  completely  overcoming  this 
difficulty. 

The  Bradford  Breaker.  The  Bradford  breaker 1  was  patented 
in  1873  by  Hezekiah  Bradford,  of  Reading,  Penn.  He  had  in 
view  the  breaking,  sizing  and  preliminary  cleaning  of  anthracite 
coal,  without  causing  the  excessive  quantity  of  fines  produced 
by  the  toothed  rolls  then  in  general  use.  However,  it  did  not 
accomplish,  on  a  commercial  scale,  what  was  proposed  on  account 
of  the  nature  of  anthracite  coal  and  intermixed  impurities.  Its 
first  employment  in  the  preparation  of  coking  coal  is  recorded 
in  1891,  when  the  St.  Bernard  Coal  and  Coke  Co.  (Earlington, 
Ky.),  the  St.  Clair  Coal  and  Coke  Co.  (Bradenville,  Penn.),  and 
the  Loyalhanna  Coal  and  Coke  Co.  (Loyalhanna,  Penn.)  used 
it  for  the  breaking,  sizing  and  preliminary  cleaning  of  coal. 

At  this  time  there  are  approximately  300  Bradford  breakers 

i  "Bradford  Coal  Breaker  and  Preliminary  Mechanical  Cleaner,"  Coal 
Age  (Vol.  15,  No.  8,  1919), 


254  COAL  WASHING 

in  active  operation  in  the  United  States;  about  75  per  cent,  of 
these  are  in  connection  with  byproduct  and  beehive  coke-oven 
plants,  the  others  for  preparing  bituminous  coal  for  producer, 
stoker  and  special  use. 

The  present  construction  and  operation  of  the  breaker  differ 
materially  from  the  first  installation  and  early  practice,  the  im- 
provements being  important  and  of  value;  while  the  original 
machine  demonstrated  the  merit  of  the  principles  involved,  yet 
its  mechanical  operation  or  action  was  not  satisfactory.  The 
principle  is  to  break  the  pure  coal  by  concussion  and  screen  it  out 
through  the  encircling  perforated  plates,  while  the  impurities, 
mingling  with  the  mined  coal,  are  generally  too  tough  to  be 
broken  by  this  force  of  concussion  and  therefore  pass  over  the 
perforations  and  out  at  the  end  of  the  machine  to  a  separate 
chute.  The  breaker  consists  of  a  large  diameter  cylinder  covered 
with  perforated  steel  plates ;  it  is  intended  to  revolve  slowly. 

The  machine  is  useful  for  preparing  coal  for  further  crushing 
and  cleaning  (Fig.  146)  ;  however,  if  the  coal,  after  passing 
through  the  perforations  of  the  breaker,  is  considered  clean 
enough  for  coking,  then  a  pulverizing  mill  is  substituted  for  the 
rolls,  so  that  impurities  intermixed  with  the  coal  will  be  powdered 
and  the  whole  product  be  of  the  same  degree  of  fineness.  The 
function  of  the  Bradford  screen  is  to  break  the  pure  coal  to  a  size 
that  will  sift  through  the  perforations,  without  breaking  much 
of  the  bony  coal,  slate,  rock  and  pyrites  intermixed  with  the 
coal ;  further,  that  these  impurities  may  be  discharged  separately. 
It  also  automatically  discards  foreign  matter  larger  than  the 
perforations,  like  car  and  rail  irons,  wooden  sprags,  and  so  on, 
that  would  be  injurious  later  to  the  rolls  or  pulverizers.  Bins 
are  provided  which  act  as  reservoirs  to  insure  a  uniform  supply 
of  coal  to  the  preparation  plant  and  as  storage  in  case  of  irregu- 
lar delivery  of  coal  from  the  mines. 

The  mechanical  feeder  not  only  regulates  the  tonnage  delivered 
to  the  screens  but  also  tends  to  balance  the  loading  of  elevators 
and  conveyors  handling  the  prepared  coal.  The  shaking  screen 
separates  the  coal  into  sizes  suitable  for  further  treatment ;  it  re- 
lieves the  breaker  of  handling  coal  already  of  the  proper  size  for 
the  rolls,  and  it  also  bypasses  around  the  rolls  the  coal  that  is  fine 
enough  for  use.  The  duty  of  the  magnetic  separator  is  to  remove 


THIl  CRUSHING  OF  COAL 


255 


256  COAL  WASHING 

pieces  of  iron,  like  bolts,  nuts,  rivets  and  mining  machine  parts, 
that  drop  through  the  perforations,  thus  preventing  serious  in- 
jury to  the  rolls  or  the  pulverizer. 

The  advantages  of  rolls  are  that  they  break  the  coal  so  as  to 
release  more  or  less  of  the  impurities  of  a  laminated  nature, 
without  pulverization;  crushing,  grinding,  pounding,  bruising 
and  pulverizing  all  tend  to  create  powder  or  dust  of  both  the 
coal  and  the  impurities;  this  latter  is  quite  an  unfavorable  con- 
dition and  hinders  success  in  either  a  dry  or  wet  method  of  coal 
preparation. 

The  hand  picking  table  is  of  use  when  the  coal  bed  contains 
bands  of  bony  or  high  ash  coal ;  the  breaker  is  then  adjusted  and 
operated  to  discard  as  much  as  possible  of  this  inferior  coal,  to- 
gether with  the  other  bulky  impurities,  as  these  coals  inferior  for 
coking  purposes  contain  much  fuel  value  for  steam  and  domestic 
use ;  the  fuel  values  are  separated  from  real  impurities  by  hand, 
while  being  conveyed  on  the  picking  table. 

In  the  general  operation  of  the  Bradford  breaker  run-of-mine 
coal  is  automatically  fed  into  one  end  of  the  cylinder  (while  re- 
volving), is  picked  up  by  longitudinal  shelves  and  dropped,  fall- 
ing on  cast  steel  "shatter  fingers"  attached  to  the  perforated 
plates;  the  coal  is  shattered  by  concussion  (see  Fig.  147),  and 
the  pieces  that  are  small  enough  sift  through  the  perforations; 
the  shelves  following  pick  up  the  large  pieces,  which  are  again 
thrown  down.  The  coal  in  falling  from  the  shelf  has  not  only 
the  force  derived  from  its  own  gravity,  but  receives  considerable 
additional  force  from  the  momentum  of  the  cylinder.  The  coal 
does  not  fall  upon  coal  in  the  lower  part  of  the  cylinder,  but  upon 
the  shatter  fingers  and  perforated  plates,  because  the  coal  is  con- 
stantly carried  toward  and  upon  the  rising  side  and  shelf.  In 
rolling  and  tumbling  around  in  the  cylinder  any  remaining 
pieces  of  pure  coal  attached  to  the  impurities  break  away  and  sift 
through  the  perforations.  The  cast  steel  shatter  fingers  not  only 
aid  in  breaking  the  coal,  but  are  so  set  on  the  inside  of  the  cylin- 
der as  to  form  a  spiral,  which  can  be  adjusted  either  to  rapidly 
advance  the  body  of  coal  to  the  opposite  end  or  to  retard  its 
progress. 

Fastened  in  the  opposite  head  of  the  cylinder  from  which  the 
coal  enters  are  refuse  removers  (wing-like  in  form)  which  aul 


THE  CRUSHING  OF  COAL  257 

matically  discard  any  impurities  or  other  matter  larger  than  the 
perforations  into  a  chute.  The  coal  sifting  through  the  perfora- 
tions on  the  revolving  screen  is  collected  underneath  the  cylinder 
and  is  conducted  by  a  chute  to  the  desired  point  for  shipment 
or  treatment. 

The  Bradford  breaker  is  adjusted  and  detail  parts  are  designed 
to  suit  the  physical  characteristics  of  the  coal  to  be  treated,  the 
requirements  of  the  market  and  the  general  conditions  and  cir» 
cumstances  of  each  individual  case.  The  capacity  and  quality 
of  work  is  affected  by  the  diameter  and  length  of  the  screen,  the 
size  of  perforations,  speed  of  rotation,  spirality  of  the  shatter 
fingers  and  the  moisture  content  and  physical  properties  of  the 
pure  coal  and  impurities.  The  difficulty  with  coal,  of  course,  is 
that  it  is  not  a  manufactured  article,  in  the  making  of  which  cer- 
tain ingredients  are  used  under  fixed  conditions  with  results  al- 
ways approximately  the  same ;  on  the  other  hand,  it  is  a  natural 
product  varying  greatly  in  quality,  often  even  in  the  same  seam, 
and  it  must  be  admitted  that  any  attempt  to  standardize  a  coal- 
preparing  machine  or  method  under  these  circumstances  would 
prove  an  almost  impossible  task. 

Coal  always  contains  a  certain  amount  of  inorganic  matter  or 
ash  which  is  intrinsically  part  of  the  coal.  The  question  as  to 
whether  one  could  clean  a  given  coal  materially  or  not  depends 
largely — essentially,  in  fact — on  the  way  in  which  that  inorganic 
matter  was  distributed  through  the  coal. 

The  rated  capacity  of  a  breaker  is  the  number  of  tons  that 
can  be  broken  sufficiently  to  sift  through  certain  size  perforations 
in  a  given  time  (usually  one  hour),  and  simultaneously  to  dis- 
charge separately  from  the  broken  coal  the  impurities  not  broken 
by  the  action  of  the  breaker.  Breakers  now  in  operation  vary 
in  capacity  from  30  tons  an  hour  (fitted  with  %  in.  perforations 
and  handling  tough  coal)  to  400  tons  an  hour  (fitted  with  3  in. 
perforations  and  handling  friable  coal). 

The  quality  of  work  done  by  a  breaker  is  shown  by  the  quan- 
tity of  pure  coal  broken  fine  enough  to  sift  through  its  perfora- 
tions, without  at  the  same  time  breaking  the  impurities  (that 
are  more  or  less  intermixed  with  the  coal)  so  small  as  to  permit 
of  their  passing  through  the  perforations,  the  impurities  being 
discharged  separately  from  the  coal.  When  making  comparisons 


258 


COAL  WASHING 


of  different  installations  regarding  the  capacity  and  quality  of 
work  done  by  breakers,  it  must  be  kept  in  mind  that  many  coals 
that  break  cubical  in  shape  (or  nearly  so)  go  through  a  screen 
quite  fast,  but  coals  that  break  in  elongated  pieces  do  not  screen 
nearly  as  fast  as  the  cubical  pieces.  Also,  when  comparing 
laboratory  tests  with  commercial  results,  it  should  be  remem- 
bered that  the  laboratory  tests  show  the  best  theoretical  results 
attainable  by  working  in  the  ways  described — such  results  repre- 
sent the  ideal  to  be  attained  by  commercial  methods. 


Fig.  147.     Bradford  Breaker 

Structurally  a  Bradford  breaker  is  a  circular  screen  of  pel 
f orated  plates  bolted  to  an  iron  framework.  Cast-iron  spidei 
or  rings,  carry  this  framework,  and  the  rings  are  rigidly  coi 
nected  to  a  central  longitudinal  revolving  shaft.  The  screen  is 
inclosed  by  a  casing  that  confines  dust  and  prevents  its  circula- 
tion throughout  the  tipple.  This  casing  protects  workmen 
against  accident  and  yet  suitable  doors  are  provided  so  that  neces- 
sary inspection  of  the  screen  and  its  operation  can  be  made.  All 
portions  of  the  screen  are  constructed  so  as  to  permit  of  ready 
renewal  or  adjustment  of. parts;  should  the  character  of  the  coal 
being  prepared  change,  or  different  marked  conditions  develop, 
then  parts  either  can  be  replaced  or  adjusted  without  serious 
disturbance  to  the  plant.  The  perforated  screen  plates  are  made 


THE  CRUSHING  OF  COAL  259 

of  extra  heavy  steel  cut  to  a  standard  size  so  as  to  fit  any  breaker 
of  the  same  diameter.  These  plates  can  have  one  size  perfora: 
tions  at  one  end  of  the  screen  and  larger  perforations  at  the 
other  end ;  this  arrangement  permits  of  two  sizes  of  coal  being 
made.  Suitable  provision  in  the  chute  to  carry  off  the  screened 
coal  keeps  the  two  sizes  separate. 

The  cast-iron  lifting  shelves  in  the  screen  are  made  of  a 
width,  length  and  angle  proportioned  to  the  work  desired  and 
the  speed  of  rotation,  so  that  the  coal  may  slide  from  the  shelves 
at  the  proper  height  to  fall  upon  the  required  part  of  the  inner 
side  of  the  screen ;  the  shelves  are  made  interchangeable  and  can 
be  replaced  or  their  position  changed  without  affecting  other 
parts  of  the  breaker.  Usually  four  shelves  are  equally  spaced 
on  the  inner  side  of  the  screen,  although  from  one  to  six  shelves 
may  be  used,  depending  on  the  coal  handled. 

The  cast-iron  refuse  removers  in  the  screen  automatically  dis- 
charge the  impurities;  generally  two  are  used  for  rapid  dis- 
charge. However,  if  a  portion  of  the  coal  is  more  or  less  tough, 
so  that  it  requires  further  tumbling  to  break  up  the  coal,  then 
only  one  remover  is  used.  In  selecting  a  breaker  it  will  be  well 
to  confer  with  those  familiar  with  the  principles  involved,  its 
construction  and  operation ;  also,  one  should  be  thoroughly  versed 
as  to  the  nature  of  the  coal  to  be  handled  and  the  separation  of 
impurities  desired.  Each  variety  of  coal  requires  special  treat- 
ment in  the  breaker,  which  can  only  be  accomplished  by  properly 
proportioning  the  parts  during  construction.  The  method  of 
operation  should  be  determined  after  a  careful  study  of  the 
physical  properties  of  the  coal.  A  knowledge  of  the  business 
end  of  the  proposition  is  essential,  as  that  often  makes  all  the  dif- 
ference between  success  and  failure.  It  is  rather  remarkable 
that  in  some  districts  collieries  which  are  in  close  proximity  to 
each  other  sometimes  differ  in  a  radical  manner  in  their  treat- 
ment of  coal  from  the  same  bed.  In  designing  a  plant  it  is  not 
always  safe  to  rely  on  a  neighbor's  experience,  or  even  on  one's 
own  experience  elsewhere.  This  uncertainty  is  .often  most  puz- 
zling and  perplexing,  but  perhaps  it  is  the  most  interesting  of 
the  various  problems  in  connection  with  mining  engineering. 

Details  of  No.  11  Test.  Below  is  given  a  record  of  five  tests 
made  with  run-of-mine  coking  coal  from  five  mines  working  the 


260 


COAL  WASHING 


RECORD  OF  TESTS  WITH  RUN-OF-MINE  COKING  COAL 


Mine 

Run-of-Mine  Coal 

CPent               C-'- 
Cent-                Ash 

Screened  Coal 
Per  Cent. 
Per                Ash  in 
Cent.            Screened 
Coal 

1 
2 
3 
4 
5 

Average, 

100 
100 
100 
100 
100 

18.70 
15.45 
16.25 
15.80 
18.22 

87.71 
86.43 
85.81 
90.17 
79.85 

14.10 
10.84 
13.00 
13.30 
13.72 

13.00 

100 

16.88 

86.00 
TABLE  40 

Breaker  Refuse 


Per 
Cent. 


12.29 
13.57 
14.19 
9.83 
20.15 

14.00 


Per  Cent 
Ash  in 
Refuse 

51.30 
44.65 
35.70 
38.40 
36.00 

41.21 


same  coal  seam  with  a  Bradford  coal  breaker  9  ft.  in  diameter 
and  11  ft.  long.  Perforations  a/£-in.  square,  peripheral  speed  425 
ft.  per  min.  Feed  60  net  tons  per  hour.  The  coal  was  quite 
soft  and  dry,  the  rock  and  bony  tough  and  the  slate  brittle. 

The  foregoing  tests  show  that  discarding  14  per  cent,  of  the 
impurities  in  the  run-of-mine  coal  larger  than  the  %-in.  perfora- 
tions reduced  the  ash  content  from  16.88  per  cent,  to  13  per  cent., 
which  was  not  acceptable  for  furnace  coke.  The  screened  coal 
from  the  five  tests  was  washed,  with  the  results  shown  below : 


Kind 

Per  Cent. 
Run-of-Mine 
Coal 

Per  Cent, 
of  Kind 

Per  Cent. 
Ash 

Breaker  screened  coal  
Washed  coal  ... 

86.00 
73.48 

100.00 
85  44 

13.00 
9  13 

Washer  refuse   

.    .  .            12.52 

14  56 

3571 

The  foregoing  results  indicate  that  to  reduce  the  ash  content 
from  16.88  per  cent,  in  the  run-of-mine  coal  to  9.13  per  cent. 
in  the  washed  coal  it  was  necessary  to  discard  a  total  of  26.52 
per  cent,  of  the  run-of-mine  coal  as  breaker  and  washer  refuse. 

When  only  3-in.  screenings  are  to  be  crushed,  one  set  of  crush- 
ing rolls  will  be  sufficient,  but  if  run-of-mine  is  to  be  crushed  for 
coking  coal,  it  is  advisable  to  crush  in  two  stages.  (1)  From 
run-of-mine  to  3  in.  in  a  crusher  having  toothed  rolls  and  (2) 
from  3  in.  to  the  desired  size  in  a  crusher  having  either  cor- 
rugated rolls  or  rolls  with  waffle-iron  teeth. 

In  either  case  a  magnetic  separator  ought  to  be  placed  ahead 
of  the  first  crusher  to  catch  all  tramp  iron  and  a  feeder  to  reg- 


THE  CRUSHING  OF  COAL  261 

ulate  the  stream  of  coal  coming  to  the  machine.  It  would  also 
be  a  good  investment  to  install  ahead  of  the  crusher  a  small 
equalizing  bin,  as  the  coal  comes  from  the  mine  in  pit  car  lots 
at  intervals.  This  will  reduce  the  power  required  to  drive  the 
crushers  decidedly.  It  is  also  important  to  install  screens  in 
front  of  each  crusher,  that  will  by-pass  all  coal  finer  than  the 
crusher  is  set  to  make.  These  screens  must  be  of  ample  dimen- 
sions to  secure  the  screening  out  of  all  the  fine  coal.  Roll 
grizzlies — if  long  enough — will  answer  the  purpose  if  the  coal 
to  be  crushed  is  dry.  With  wet  coal  the  grizzlies  clog  up  and 
act  only  as  conveyors. 

If  close  crushing  is  required  and  oversize  objectionable,  a  siz- 
ing screen  ought  to  be  installed  after  the  final  crushers.  The 
oversize  from  this  screen  must  be  returned  to  the  crusher  and 
remains  in  a  close  circuit  until  reduced  to  the  proper  size  to 
pass  through  the  screen. 


CHAPTER  XXVII 

CRUSHING  AND  RE-WASHING  OF  THE  MIDDLE- 
PRODUCTS 

When  retreating  middle  products  the  materials  to  be 
crushed  consist  of  washed  but  intergrown  coal  from  %6  in. 
to  3  in.  in  size.  The  purpose  of  crushing  is  to  break  up  the 
intergrown  particles  in  such  a  way  that  the  slate  or  bone  adher- 
ing to  or  disseminated  throughout  the  coal  shall  be  mechanically 
separated  from  it.  This  will  give  a  homogeneous  mass  of  ma- 
terial suitable  for  rewashing. 

Two  considerations  must  be  taken  into  account.  The  char- 
acter of  the  middle  product  influences,  at  least  in  a  theoretical 
way,  thet  crushing  process.  The  most  suitable  method  would 
be,  without  doubt,  to  spall  off  the  slate  and  bone  without  crush- 
ing them.  This  would  make  the  subsequent  water  clarification 
and  sludge  recovery  much  easier.  But  the  foregoing  procedure 
would  only  be  possible  if  the  impurities  are  attached  to  the  coal 
as  shown  in  Fig.  148.  This  condition  occurs  only  in  solid  coal, 


Fig.  148 

where  the  impurities  are  to  be  found  at  the  cleavage  line  be- 
tween the  bed  of  coal  and  the  roof  or  the  bottom.  In  most 
cases,  however,  we  must  consider  impurities,  which  are  more  or 
less  disseminated  throughout  the  coal  mass,  as  shown  in  Fig.  149. 
In  this  case  a  crushing  of  the  impurities  cannot  be  avoided. 
(2)  In  every  case  the  degree  of  crushing — that,  is,  the  difference 
between  the  size  of  the  material  before  and  after  crushing — 
must  be  made  as  small  as  possible.  The  crusher  giving  the  great- 
est yield  with  the  smallest  amount  of  sludge  is  the  one  most  suit- 

262 


RE-WASHING  (>/•'  MIDDLE  PRODUCTS 


263 


able,  because  this  permits  the  most  perfect  separation.  Whether, 
in  each  separate  case,  the  yield  or  the  amount  of  sludge  should 
be  considered  more  important  depends  on  the  one  hand  upon 
the  difficulty  of  water  clarification  and  on  the  other  hand  upon 
the  value  of  the  fine  coal. 

The  crushing  plant  ought  to  be  located  between  the  primary 
and  rewash  jigs.  This  will  necessitate  the  installation  of  an  ele- 
vator, because  all  the  jigs  should  be  located  upon  one  platform 
at  the  same  level. 

The   crushing  of  the  middle  products  can  be  accomplished 


Fig.  149 

either  with  roll  crushers,  gyratory  crushers  or  disintegrators. 
Roll  crushers  can  have  either  smooth  or  slightly  corrugated 
rolls.  Such  crushers  will  permit  the  free  passage  of  flat  pieces 
of  slate,  which  affords  a  decided  advantage  over  all  other  types. 
For  crushing  down  to  ^ic-in.  size,  roll  crushers  are  well  adapted. 
If  the  middle  product  should  be  crushed  still  finer,  gyratory 
crushers  or  disintegrators  are  advisable.  Gyratory  crushers  can 
be  used  down  to  Me  in.  size,  and  for  still  finer  crushing  disinte- 
grators should  be  employed. 

CRUSHER  DATA 


Type 
Roll  crusher   

Crushing                    Capacity  per            Power 
From                          To       Hour  in  Tons         Required 
3  in.               %o  in.             6  to  60         3  to    50  h.p. 
3  in.               Vio  in.              1  to  12        2  to    20  h.p. 
3  in.             65  mesh           3  to  85        2  to  150  h.p. 

Gyratory  crusher 
Disintegrators    

TABLE  41 


The  process  of  rewashing  does  not  differ  materially  from 
the  process  used  in  primary  washing.  The  regulation  of  the 
rewash  process  depends  upon  the  purpose  for  which  the  resulting 
products  will  be  used.  If,  according  to  the  character  of  the 


264  COAL  WASHING 

middle  product,  only  boiler-house  coal  can  be  made,  close  wash- 
ing is  not  necessary  and  the  rewashing  should  be  carried  on  in 
such  a  way  that  the  resulting  refuse  will  be  as  free  from  good 
coal  as  possible.  If,  however,  the  middle  product  is  of  such  a 
nature  that  the  pure  coal  contained  therein  is  good  enough  to 
be  mixed  with  the  primary  washed  coal,  the  middle  product 
should  be  washed  very  closely.  In  some  cases  it  would  be  more 
economical  to  rewash  for  boiler-house  coal  only,  even  though 
the  product  of  the  rewash  jigs  could  be  made  clean  enough 
to  be  mixed  with  the  primary  washed  coal,  on  account  of  the 
great  loss  of  good  fuel  in  the  refuse,  imposed  by  trying  to  get 
a  perfectly  clean  washed  product. 


CHAPTER  XXVIII 
DE-WATERING  AND  STORAGE  OF  FINE  COAL 

Washed  coal  must  be  freed  from  adhering  moisture  before 
it  can  be  shipped  to  market.  Coal  larger  than  }£  in.  can  be 
dewatered  easily  by  simply  passing  it  over  draining  screens, 
but  the  dewatering  of  finer  sizes  is  a  different  problem  and  the 
methods  used  at  present  do  not  give  entirely  satisfactory  results. 
We  should  not  overlook  therefore  any  efforts  for  further  de- 
velopment and  improvement  in  the  process  of  dewatering  the 
fine  coal. 

Before  we  can  discuss  intelligently  the  methods  used  at 
present,  we  must  first  determine  the  purpose  of  the  dewatering 
process  and  the  scope  of  the  demands  made  by  it  upon  the  ap- 
paratus used.  The  final  purpose  of  dewatering  is  to  produce 
a  coal  of  the  highest  possible  value.  This  will  permit  us  to  pre- 
determine in  each  separate  case  the  most  economical  degree  to 
which  the  dewatering  should  be  carried.  Some  typical  cases 
are  as  follows: 

Coking  Coal.  A  moisture  content  of  from  4  to  6  per  cent,  is 
the  most  suitable  for  the  coking  process  in  retort  ovens  when 
utilizing  the  by-products.  Therefore  the  coal,  if  the  character 
and  size  will  permit,  must  be  dewatered  to  this  extent.  If  this 
is  not  possible,  other  means  must  be  employed  to  help  out.  Dry- 
screened  dust  may  be  mixed  in  or  even  dry-screened  fine  coal. 
The  amount  of  the  unwashed  coal  which  can  be  thus  mixed  in 
depends  upon  the  percentage  of  ash  it  contains. 

Fuel  Coal.  The  degree  of  dewatering  of  fine  coal  depends 
upon  the  demands  of  the  consumer,  but  the  moisture  should  not 
exceed  10  per  cent.  Mixing  in  of  dry  unwashed  fines  will  also 
be  of  some  benefit,  but  the  recrushing  of  coarse  coal  for  this  pur- 
pose should  be  avoided  ordinarily  on  account  of  the  greater 
value  of  the  coarser  sizes. 

The  following  may  be  considered,  taking  into  account  the  diffi- 

265 


266  COAL  WASHING 

culties  of  dewatering  and  the  rapid  increase  of  these  difficulties 
with  any  decrease  of  the  moisture  in  the  final  product.  As  much 
as  the  conditions  permit,  the  drying*  of  the  fine  coal  should  be 
aided  by  the  mixing  in  of  dry  raw  coal. 

In  most  cases  greatest  possible  dryness  of  the  coal  is  required. 
The  requirements  of  this  dryness  should  be  established  before- 
hand by  a  guaranty  in  regard  to  the  permissible  upper  limits  of 
moisture  in  the  final  product,  so  that  the  washery  as  well  as  the 
consumer  may  have  fixed  data  to  go  by. 

Simplicity  of  installation  demands  the  smallest  possible  space, 
low  power  consumption  and  small  cost  of  installation  and  opera- 
tion. The  dewatering  of  the  fine  coal,  appearing  at  first  sight 
to  be  easy,  thus  becomes  a  difficult  problem  made  more  difficult 
by  the  inclination  of  the  fine  coal  to  pack  together  in  dense  cakes 
containing  a  high  amount  of  water. 

The  continuous  stream  of  coal  coming  from  the  mine  docs 
not  allow,  except  at  high  cost,  the  devoting  of  much  time  to  any 
one  separate  stage  of  its  preparation.  One  process  must  follow 
another  without  appreciable  intervals  or  interruptions.  Even  in 
the  storage  bins  the  coal  does  not  remain  for  any  length  of  time. 
It  must  be  loaded  out  continuously.  A  coal  washery  knows  only 
the  following  alternative — few  swiftly  operating  pieces  of  ap- 
paratus or  a  great  number  of  slower-working  machines.  For  all 
previously  enumerated  apparatus  the  principle  of  quick  opera- 
tion is  easily  accomplished;  the  treatment  of  fine  coal  offers 
serious  difficulties  which  still  remain  to  be  solved  satisfactorily. 

The  methods  to  be  employed  for  drying  coal  must  be  adapted 
to  the  character  of  the  material.  This  requirement  demands 
especial  consideration.  It  is  impossible  to  prefer  one  method 
above  all  others  at  first  sight.  The  character  of  the  fine  coal 
from  different  mines  shows  many  variations.  With  a  hard, 
not  easily  shattered  slate  the  fine  coal,  and  especially  the  sludge, 
are  innocuous.  The  dewatering  is  comparatively  easy  and  can 
be,  at  least  partly,  combined  with  the  water  clarification  process. 
But  if  the  slate,  or  what  is  even  worse,  the  slate  and  coal  are  dis- 
posed to  produce  a  microscopically  fine  pulp  held  in  suspension 
in  the  water,  the  process  of  dewatering  must  be  carried  on  in  an 
entirely  different  manner.  The  separation  of  the  fine  coal  froi 


DE-WATERING  OF  FINE  COAL  267 

the  pulp  must  be  accomplished  in  the  early  stages  of  the  process 
if  it  is  to  be  carried  out  successfully. 

Methods  of  Drying.  Considering  the  requirements  set  forth 
we  have  the  following  methods  for  drying  in  use  at  the  present 
time:  (1)  Dewatering  in  bins  or  pits ;  (2)  dewatering  on  slowly 
moving  conveyors ;  (3)  centrifugal  dryers ;  (4)  niters  (for  sludge 
only). 

De-watering  Pits.1  The  attractive  features  of  dewatering  pits 
are:  (1)  A  drained  washed  coal  containing  8  to  9  per  cent, 
moisture;  (2)  a  filtered  water  free  from  sediment,  for  recircu- 
lation;  (3)  no  escape  of  dirty  water  (to  pollute  private  and 
public  streams)  except  that  small  amount  evaporating  and  ad- 
hering as  external  moisture  to  the  coal;  (4)  a  rapid  filtration  of 
the  water  so  as  to  gain  a  brimful  pit  of  coal,  without  shifting  the 
stream  of  water  and  coal  from  the  washing  machines;  (5)  uni- 
formity of  the  drained  mass  of  coal,  in  regard  to  fine  and  coarse 
sizes,  sludge  and  pieces  of  various  characteristics  being  evenly 
distributed  so  that  a  homogeneous  coking  coal  may  be  gathered ; 
(6)  no  mechanical  power  necessary  to  aid  or  hasten  the  dewater- 
ing of  the  coal;  (7)  permanent  construction  with  only  a  mini- 
mum maintenance  expense;  (8)  an  economically  operated  ap- 
paratus for  removing  the  drained  coal. 

The  principle  involved  in  the  drainage  of  coal  in  a  pit  is 
that  water  moves  gradually  in  a  wavering  descending  motion 
through  a  mass  of  minute  particles  at  rest,  and  is  clarified  during 
this  movement.  The  dewatering  capacity  of  a  pit  depends  upon 
the  relation  between  the  number  of  square  feet  of  filtering  sur- 
face to  the  quantity  of  water  delivered  in  a  given  time  and  the 
fineness  of  the  coal. 

Tests  made  at  five  coal  washeries  using  dewatering  pits  indi- 
cate that  the  average  filtering  capacity  of  a  pit  is  32  gal.  of  water 
per  hour  for  each  square  foot  of  filtering  surface,  when  dewater- 
ing coal  in  sizes  ranging  from  }£-in.  cubes  to  dust,  and  the  pit 
will  be  filled  brimful  of  coal  without  teinpora^  cessation  to 
prevent  water  overflowing  the  pit  walls.  Most  careful  moisture 
determinations  were  also  made  of  the  drained  washed  coal  at 
different  times  as  gathered  from  the  pits  for  coking  after  various 
i  "Dewatering  Pits  for  Washed  Coal,"  Coal  Age  (Vol.  14,  Xo.  24,  1918). 


268 


COAL  WASHING 


hours  of  drainage,  with  the  results  shown  below,  the  drained 
washed  coal  averaging  in  fineness  ^-in.  cubes  to  dust. 


Pits   2 

;4  Ft.  Deep 

Average 
Moisture, 
per  cent. 

Maximum 
Moisture, 
per  cent. 

Minimum 
Moisture, 
per  cent. 

After  24 

hours' 

drainage  

9  15 

10.36 

7.37 

After  36 

hours' 

drainage  

9  05 

10.61 

7.10 

After  48 

hours' 

drainage   

8.26 

10  13 

7.00 

After  00 

hours' 

drainage  

805 

9.74 

6.84 

After  72 

hours' 

drainage.  . 

7.97 

9.48 

6.18 

Pits   16   Ft.   Deep 
After  12  hours'  drainage 
After  24  hours'  drainage 
After  36  hours'  drainage 
After  48  hours'  drainage. 
After  60  hours'  drainage, 
After  72  hours'  drainage 


10.05 
8.46 
8.39 
7.90 
7.65 
7.60 


12.96 
9.07 
8.60 
8.31 
8.00 
779 


8.16 
8.06 
8.03 
7.76 
6.85 
6.90 


TABLE  42 


The  pits  are  formed  by  rectangular  concrete  walls  of  a  thick- 
ness suitable  to  withstand  the  pressure  when  the  pits  are  filled 
with  coal  and  water.  The  ultimate  concrete  bottom  is  laid 
slightly  sloping  toward  centrally  located  drains  which  transfer 
the  filtered  water  to  a  pump  sump  for  repeated  use.  Above  the 
ultimate  bottom,  leaving  an  intervening  space  of  4  in.,  a  filtering 
platform  is  placed.  This  contains  small  V-shaped  gaps  for  sup- 
porting the  coal  and  allowing  the  passage  of  the  dripping  filtered 
water  onto  the  concrete  bottom.  In  practice  the  final  8  to  12  in. 
in  depth  of  coal  is  not  drawn  off  but  remains  on  the  filtering 
platform  as  a  permanent  filter  bed.  This  bed  should  be  stirred 
up  frequently,  say  after  each  third  tilling  of  the  pit,  by  the  ap- 
pliance provided  on  the  traveling  coal  excavator,  so  as  to  pre- 
sent a  more  nearly  perfect  porous  bed,  in  order  that  the  filter- 
ing capacity  of  the  pit  may  not  become  diminished. 

The  permanency  of  properly  designed  and  constructed  con- 
crete pits  is  unquestioned.  They  do  not  deteriorate  in  useful- 
ness or  value  by  constant  usage.  The  only  renewal  necessary 
is  the  replacement  of  the  false  bottom  used  as  a  filtering  plat- 
form in  two  to  four  years,  according  to  usage.  This  platform 
is  built  in  sections  of  about  4x6  ft.,  so  that  one  or  more  sections 
may  be  readily  replaced  when  necessary. 

The  cleaned  coal  and  black  water  from  the  washing  machines 


I 


DE-WATERING  OF  FINE  COAL  269 

is  delivered  directly  to  the  pits  by  means  of  extension  gravity 
sluiceways  swung  over  the  top  of  the  pit  so  that  the  latter  will 
be  uniformly  filled  with  coal.  The  separation  of  the  water  from 
the  coal  require  no  machinery,  attendance  or  supplies.  To  save 
the  water  draining  from  pits  when  the  washing  machines  are 
not  in  operation,  concrete  reservoirs  of  sufficient  capacity  are 
provided  with  drains  from  the  pits  and  to  a  pump  sump,  so 
that  all  water  will  be  saved  for  recirculation  when  the  wash- 
ing machines  are  again  operated.  As  there  are  no  drains 
leading  from  any  part  of  the  drainage  system  to  sewers  or 
ditches,  there  is  no  chance  to  pollute  streams  with  water  holding 
impurities  in  solution  or  suspension. 

There  are  no  mechanical  dewatering  units  between  the  wash- 
ing machines  and  the  drainage  pits.  The  problem  of  sludge 
and  dirty  water  disposal  is  thus  entirely  eliminated,  as  all  fine 
coal  and  sludge  is  intermixed  with  the  drained  washed  coal. 
This  method  of  dewatering  results  in  a  great  saving  of  water, 
power,  labor  and  upkeep  over  the  ordinary  expensive  dewater- 
ing methods  employing  centrifugal  driers  or  perforated  bucket 
elevators,  with  sludge  tanks  and  a  multiplicity  of  machinery. 
Whenever  these  are  in  use  great  fields  of  fine  coal  and  sludge 
are  visible  with  the  accompanying  pollution  of  near-by  streams. 

The  fines  to  be  dealt  with  consist  of  particles  capable  of  pass- 
ing through  extremely  fine  meshes,  the  greater  part  of  them 
being  from  %  in.  to  Vioo  in.  in  diameter,  or  say,  from  64  to 
10,000  to  the  square  inch. 

In  table  43  is  shown  the  number  of  square  feet  of  pit  filtering 
surface  necessary,  when  washing  coal  at  the  rate  of  200  net  tons 

MOISTURE  DETERMINATIONS  AFTER  VARIOUS  HOURS  OF  DRAINAGE 


Various    Quantities    of    Water 
•D  4.-       i  ITT  i                   Necessary    to    Wash    1    Net 
Ratio  of  Water  to            Ton  of  Crushed  Raw  Coal 
1  Ton  of  Coal 
In                 In                   In 

Net  Tons 
of 
Crushed 
Raw  Coal 
Washed 

Square 
Feet 
of  Pit 
Filtering 
Surface 

Cu.  Ft. 

Gal. 

L,b. 

per  Hour 

Necessary 

1 

32 

240 

2,000 

200 

1,500 

m 

48 

360 

3,000 

200 

2,250 

f> 

64 

480 

4,000 

200 

3,000 

2'/> 

80 

600 

5,000 

200 

3,750 

3      . 

96 

720 

6,000 

200 

4,500 

3V2    .  .  . 

112 

840 

7,000 

200 

5,250 

TABLE  43 


270  COAL  WAXHIXC 

per  hour,  with  jigs  that  require  varying  quantities   of  water 
in  order  to  wash  one  net  ton  of  raw  crushed  coal. 

The  ratio  of  water  to  that  of  coal  is  determined  by  the  water 
requirements  of  the  particular  coal-washing  machine  under  con- 
sideration. One  washery  handling  200  tons  of  coal  per  hour, 
adopting  a  specific  form  of  washing  machine,  may  need  water  in 
the  proportion  of  1M>  to  1  of  coal,  requiring  only  2250  sq.  ft.  of 
pit  filtering  surface;  while  another  washery  handling  the  same 
tonnage  per  hour  using  another  form  of  washing  machine  may 
need  water  in  the  proportion  of  3  to  1  of  coal,  necessitating  4500 
sq.  ft.  of  pit  filtering  surface. 

When  considering  the  adoption  of  dewatering  pits,  it  is  im- 
portant to  study  the  water  requirements  of  coal-washing  ma- 
chines and  to  see  that  a  machine  requiring  an  extravagant  quan- 
tity of  water  is  not  selected,  since  it  would  demand  dewatering 
pits  of  extreme  and  excessive  dimensions.  As  aforementioned, 
the  series  of  tests  made,  relating  to  the  filtering  capacity  of  pits, 
demonstrated  that  the  drainage  capacity  averaged  32  gal.  of 
filtered  water  discharged  per  hour  for  each  square  foot  of  filter- 
ing surface,  through  coal  ranging  from  J/£-in.  cubes  to  dust. 

To  ascertain  the  floor  dimensions  of  a  pit  to  hold,  say,  1000 
net  tons  of  drained  coal,  or  40,000  cu.  ft.,  weighing  on  a  dry 
basis  50  Ib.  per  cubic  foot,  assuming  that  water  will  enter  the 
pit  with  the  coal  for  five  consecutive  hours,  the  coal  coming  at 
the  rate  of  200  tons  an  hour,  each  ton  requiring  360  gal.  of 
water  for  washing,  the  water  filtering  away  at  the  rate  of  32 
gal.  per  hour  for  each  square  foot  of  filtering  surface,  the  for- 
mula is  as  follows: 

Let 

a  =  The  number  of  gallons  of  water  required  to  wash  one 

of  coal; 

Z>  =  Tons  of  coal  delivered  to  pit  per  hour; 
c  =  Gallons  of  water  filtering  away  per  hour,  per  square  fc 

of  pit  surface; 

#  =  Square  feet  of  pit  surface  required. 
Then  al) 


DE-WATERING  OF  FINE  COAL  271 

Example — a  =  360  gal.  of  water  per  ton  of  coal ; 
&  =  200  tons  of  coal ; 

c  =  32  gal.   of  water   per  hour  per  square   foot  of 
draining  surface. 

Then  360  X  200  -r-  32  =  x,  or  2250  sq.  ft.  of  pit  floor  surface 
required.  This  would  be  equivalent  to  a  pit,  say,  30  ft.  wide  by 
75  ft.  long. 

The  formula  for  determining  the  height  of  pit  walls  is  as 

follows : 

Let 

d  =  Square  fee't  of  pit  surface  required ; 
e  =  Cubic  feet  of  coal  required ; 
x  =  Height  of  walls  in  feet ; 

e 


Example — d  =  2250  sq.ft.  of  pit  floor  surface; 

e  =  40,000  cu.  ft.  of  coal. 
40,000  —  2250  =  x,  or  17  ft.  10  in.  =  the  height  of  pit  walls. 

To  offset  the  8  in.  to  12  in.  in  depth  of  coal  used  as  a  filter 
bed  and  the  height  of  filter  platform  above  the  concrete  floor, 
about  20  in.  must  be  added  to  the  height  of  walls,  to  acquire 
a  working  capacity  of  1000  tons  of  coal.  This  will  make  a  pit 
averaging,  say,  30  ft.  wide,  75  ft.  long  and  19  ft.  high  above 
the  concrete  floor. 

A  pit  to  drain  coal  from  a  washery  using  720  gal.  or  double 
the  quantity  of  water  assumed  above  as  being  necessary  to  wash 
a  ton  of  coal,  would  require  a  filtering  surface  of  4500  sq.  ft.  to 
receive  the  coal  and  water  in  five  consecutive  hours  at  the  rate 
of  200  tons  of  coal  per  hour.  This  would  mean  a  pit,  say,  45 
ft.  wide,  100  ft.  long  and  with  a  total  depth  of  10  feet  6  inches. 

A  pit  this  size  would  not  be  economical  in  ground  space,  con- 
struction or  in  handling  the  coal  and  water  to  and  from  it.  De- 
watering  pits  are  not  practical  to  operate  in  connection  with 
washeries  using  an  excessive  quantity  of  water  to  wash  a  ton  of 
coal. 

The    table    showing    moisture    determinations    after    various 


272 


COAL  WASHING 


hours  of  drainage  indicates  that  after  24  hr.,  additional  drainage 
time  does  not  reduce  the  moisture  content  of  the  coal  materially, 
the  water  draining  off  rapidly ;  and  with  the  exception  of  a  few 
feet  at  the  top,  no  benefits  of  air  circulation  are  secured  to  aid 
in  reducing  the  moisture.  The  coal  is  so  closely  packed  by  the 
water  action  that  trenches  formed  by  slides  30  in.  wide  and 
4  to  6  ft.  deep  do  not  cave  in. 

The  drained  coal  is  gathered  from  the  pits  by  a  crane,  hav- 
ing a  vertical  and  horizontal  motion.  This  is  mounted  on  wheels 
and  provided  with  numerous  buckets  for  excavating  and  elevat- 
ing the  coal  to  a  belt  conveying  system  that  discharges  it  into 
larry  bins  for  use  in  coking.  The  crane  spans  the  pits,  and  its 
movements  are  guided  by  rails  fastened  to  the  top  of  the  longi- 
tudinal pit  walls.  The  elevating  capacity  is  from  400  tons  per 
hour  up. 

Draining1  Bins.  The  dewatering  of  the  fine  coal  is  also  ac- 
complished to  some  degree  in  the  commonly  used  storage  bins. 
A  storage  of  48  hr.  will  reduce  the  moisture  in  the  coal  to  from 
10  to  12  per  cent.  In  Europe  draining  bins  are  commonly  em- 
ployed and  the  draining  off  of  the  water  is  accelerated  by  the 
use  of  filter  bodies  made  of  expanded  metal,  which  open  up  the 
densely  packed  mass  of  fine  coal.  The  following  results  have 
been  obtained  with  this  type  of  bin : 


Capacity 
of 
Washery 
in  Tons 
per  Hour 

Contents 
of  Bins 
in  Tons 

Time  Required         Capacity  of 
Number    Filling-Dewatering        All  Bins 
of  Bins            of  One  Bin                in  Tons 
in  Hours                 per  Hour 

Degree  of 
Moisture 
in  the  Dried 
Coal, 
per    cent. 

100 
150 
200 

600-1200 
1200-2000 
1400-3000 

4-12 
8-20       2-6          20-48         20-120 
10-24 

8-13 

TABLE  44 

The  disadvantages  of  draining  bins  are  as  follows:  On 
count  of  the  large  surfaces  the  sludge  settles  out  of  the  watei 
considerably  delaying  thereby  the  process  of  dewatering. 
account  of  the  lack  of  other  drying  apparatus,  all  sludge  pi 
duced  must  be  sluiced  into  the  draining  bins,  there  to  be  d< 
watered.  This  delays  also  the  rapid  draining  off  of  the  watei 
In  emptying  the  bins,  the  coarse  coal  flows  out  more  rapid! 


DE-WATERING  OF  FINE  COAL 


273 


than  the  fine  coal  and  the  sludge,  which  later  clings  to  the  walls. 
When  the  bins  are  emptied  this  sludge  hangs  to  the  walls  for 
some  time  and  drops  off  suddenly  in  large  masses.  This  destroys 
that  uniformity  of  the  coal  which  is  desirable  for  the  coking 
process.  The  bins  also  require  considerable  space  in  all  direc- 
tions, and  if  the  ground  area  at  disposal  is  limited  it  will  bring 
about  a  cramped  or  less  desirable  arrangement  of  the  other 
apparatus. 

To  prevent  dripping  of  the  water  on  the  floor  below  the 
bins,  special  gates  are  employed,  which  permit  the  water  to  flow 
out  at  one  side  where  it  can  be  carried  away  in  pipes  or  open 
troughs.  Fig.  150  shows  the  construction  of  such  a  gate.  The 


Fig.  150.     Draining  Gate  Under  Washed  Coal  Bins 

slide  is  being  pressed  against  the  gate  seat  by  the  eccentric  lever 
"b"  and  the  water  flows  through  the  screen  mantle  "e"  into 
a  pipe  "d."  The  gate  can  be  opened  by  a  rack  and  pinion  "f " 
operated  by  a  handwheel  "g."  This  arrangement  keeps  the 
floor  under  the  draining  bins  clean  and  dry  and  the  outflowing 
water  can  be  collected  and  returned  to  the  system. 

Draining  conveyors  work  on  quite  a  different  principle.  They 
dewater  the  fine  coal  on  its  way  to  the  storage  bins.  No  special 
dewatering  device  is  necessary,  and  the  conveying  apparatus  re- 
quired in  any  case  is  adapted  to  dewatering  the  coal.  Convey- 
ors or  elevators  can  be  used  for  this  purpose,  depending  upon 
the  juxtaposition  of  the  jigs  to  the  storage  bins.  When  these 
machines  are  employed  the  washed  coal  can  be  sluiced  from  the 
jigs  directly  to  the  conveyors.  With  elevators  the  coal  must  be 
sluiced  into  a  settling  tank  out  of  which,  the  elevators  feed. 


274 


COAL  WASHING 


The  drained-off  water,  carrying  fine  particles  of  coal  in  suspen- 
sion, is  sluiced  into  separate  clearing  tanks.  Dewatering  eleva- 
tors must  be  built  heavy,  depending  upon  the  character  of  the 
coal,  the  required  capacity,  and  the  distance  over  which  the 
material  must  be  conveyed.  This  is  the  more  important  since 
the  speed  of  the  conveyors  must  be  slow  in  order  to  give  the 
water  time  to  drain  off. 

The  following  table  gives  some  data  on  de watering  elevators 
and  conveyors : 


Type 

Dimensions 
Width                 Length 

Slope 
in 
Deg. 

» 

per 
Minute 

Capacity 
per 
hour 
in  Tons 

Power, 
Hp. 

De- 
watered 
to 
per  cent. 
Moisture 

Dewatering 

conveyor    32  in.-13  ft.     50-130  ft.       0-40     1%-12       5-60       4-18     10-13 
Dewatering 

elevator     20  in.-  6  ft.     50-130  ft.     40-65     3    -32     10-60     12-32     10-13 


TABLE  45 

Centrifugal  Dryers.  Centrifugal  dryers,  on  account  of  their 
high  speed,  are  restricted  in  regard  to  the  dimension  of  the 
diameter  of  the  revolving  parts.  To  accomplish  a  satisfactory 
capacity  only  centrifugals  with  continuous  feeds  and  discharge 
can  be  considered.  At  present  only  two  types  of  these  machines 
are  in  use.  In  one  the  dried  coal  is  discharged  continuously, 
being  scraped  off  the  screen  plates  by  knives  which  rotate  at  a 
speed  different  from  that  of  the  screens.  In  the  other  type 
scrapers  are  not  used  and  the  coal  is  discharged  from  the 
screens  through  trapdoors  which  open  and  close  intermittently. 

The  Elmore  Continuous  Centrifugal  Dryer  has  many  advan- 
tages over  the  so-called  " batch"  type  of  machine,  chief  among 
which  are  its  greater  capacity  and  its  economy  in  power  con- 
sumption. For  certain  substances,  the  increase  in  capacity  is 
as  much  as. twenty  to  one.  The  reduction  in  power  consump- 
tion is  due  to  the  continuous  rotation  of  the  high  speed  parts,  as 
opposed  to  the  frequent  stopping  and  starting  of  the  "batch" 
type,  which  occurs  many  times  within  an  hour.  This  in- 
creased capacity  and  saving  in  power  results  in  large  economy 
in  the  cost  of  the  plant  and  its  operation,  as  well  as  the  saving 
in  floor  space,  power,  and  labor  required  for  operation. 


DE-WATERING  OF  FINE  COAL 


275 


Structure  and  Operation  of  Elmore  Continuous  Centrifugals. 

These  machines  are  built  in  two  types — Style  A  (Fig.  151)  and 
Style  B  (Fig.  152)  Fig.  153  shows  a  style  A  dryer  with  one 
screen  segment  removed,  showing  scrapers  and  distributing  cone. 
Style  A  is  driven  from  above  and  Style  B  from  below.  Style 


Fig.  151.     Elmore  Centrifugal  Dryer.     Style  "A" 

A  can  be  driven  by  a  counter-shaft,  and  rawhide  bevel  gear, 
as  shown  in  Fig.  154,  or  by  direct  motor  drive  connected  to  the 
counter-shaft,  or  vertically  mounted,  as  shown  in  Fig.  156. 
With  direct  motor  drive,  flexible  couplings  are  used  and  speed 


276 


COAL  WASHING 


reducing  gears  of  approved  type  are  furnished  to  accommodate 
high  speed  motors. 

The  structure  of  the  Style  A  machine  is  exceedingly  rigid, 
the  base  alone  weighing  over  6000  Ibs.  Power  is  applied  to  the 
central  shaft  (3)  which  is  carried  with  its  weight  and  all  that  is 
attached  to  it,  on  the  set  of  ball  bearings  (4)  resting  on  top 


Fig.   152.     Elmore  Centrifugal  Dryer.     Style  "B" 

of  the  gear  case  (5).  The  main  shaft  (3)  is  guided  only  by  the 
bearing  (24)  at  its  lower  end  and  lubricated  through  pipe  (25). 
At  the  lower  end  of  shaft  (3)  is  keyed  a  cast  steel  spider  (22) 
and  on  its  outer  rim  rests  the  truncated  conical  basket  or  screen 
frame  (23)  made  in  eight  segments  bolted  together.  To  the  in- 
side of  each  segment  is  secured  a  suitable  screen,  the  character 
of  the  perforations  depending  on  the  material  to  be  treated. 


DE-WATERING  OF  FIXE  COAL 


277 


The  shaft,  spider  and  screen  frame  (3,  22  and  23)  have,  there- 
fore, a  common  support  at  the  bearing  (4)  and  a  common  rate  of 
rotation.  Near  the  upper  end  of  the  shaft  (3)  is  keyed  a  steel 
cut  gear  (7)  and  by  the  four  gears  (7,  8,  9  and  10)  power  is 
transmitted  to  the  hollow  or  quill  shaft  (12)  which  surrounds 
the  main  shaft  (3).  The  gear  (8)  is  keyed  to  the  extended  hub 
of  the  gear  (9)  which  has  a  bronze  bushing,  and  both  revolve 


v 

*-* ••- ,-    m\ 


Fig.  153.     Elmore  Dryer  with  One  Screen  Segment  Eemoved 

on  the  stationary  shaft  (6)  and  their  weight  is  carried  by  the 
ball  bearing  (11).  Gear  case  (5)  is  divided  on  the  center  line 
of  the  shaft  (6)  and  access  to  these  gears  is,  therefore,  very 
simple.  Ample  opportunity  for  inspection  is  afforded  through 
the  plates  (34). 

It  will  be  noted  that  the  quill  shaft  (12)  rotates  in  the  same 
direction  as  the  main  shaft  (3).  This  rate  of  rotation  is  re- 
duced by  the  gears  from  2  to  10  per  cent.,  depending  on  con- 


278 


COAL  WASHING 


ditions,  and  its  weight  with  all  the  parts  attached  to  it,  is  car- 
ried by  the  ball  bearings  (13)  resting-  on  the  support  (14).  At 
the  lower  end  of  the  quill  shaft  (12),  is  keyed  the  distributing 


Fig.  154.     Cross  Section  Showing  Construction  of  Style  "A"  Elmore  Dryer 

cone  (21)  to  which  is  bolted  a  series  of  high-pitched,  helical 
scraping  nights  (20).  Elements  (10,  12,  20  and  21)  therefore 
rotate  as  a  unit  and  at  a  slightly  reduced  speed  from  the  units 
(3,  22  and  23).  The  effect,  therefore,  is  to  have  the  screen  on 


DE-WATERING  OF  FIXE  COAL 


279 


the  inside  of  the  screen  frame  (23)  slowly  pass  the  scraping 
flights  (20),  although  both  may  have  an  absolute  rotation  of 
several  hundred  r.p.m. 

The  process  of  extracting  the  moisture  is  simple. .  The  wet 
material  is  fed  through  the  inlet  (17),  falls  on  distributor  (19), 
then  on  cone  (21),  when  the  centrifugal  force  throws  it  horizon- 
tally to  the  conical  screen  on  the  inside  of  frame  (23).  As  the 


Fig.  155.     Elmore  Dryer  Style  "A" 

screen  slowly  passes  the  flights  (20),  the  water  goes  through  the 
screen  into  the  open  space  outside  and,  retained  by  the  casing 
(33),  falls  into  channels  cast  in  the  base  (27)  and  flows  out  at 
opening  (32).  While  the  water  is  thus  escaping  through  the 
screen,  the  solid  matter  is  being  gathered  up  in  rolls  at  each 
of  the  flights  (20),  which  are  placed  at  such  a  pitch  that  the 
material  slides  down  the  face  of  these  flights,  is  thrown  off  at 
their  lower  end  and  discharged  against  renewable  ring  (28). 


280 


COAL  WASHING 


From  this  point  it  falls  through  the  center  opening  in  the  base 
casting  and  out  of  the  machine  through  hopper  (26)  on  to  a 
conveyor  or  other  means  of  removal.  Rings  (28  and  29)  form  a 
lock  to  prevent  solids  from  passing  to  the  water  compartment 
or  vice  versa.  Inasmuch  as  a  comparatively  small  amount  of 


Fig.  156.     Elmore  Dryer  with  Direct  Motor  Drive 

material  is  being  conveyed  by  each  flight  at  any  one  time,  it  will 
be  seen  that  every  opportunity  is  afforded  for  the  water  to  free 
itself. 

After  having  read  and  thoroughly  understood  the  foregoing 
description  of  the  Style  A  machine,  the  construction  and  opera- 
tion of  Style  B  will  be  readily  seen.  The  main  points  of  differ- 
ence are  as  follows : 

First — The  differential  gears  (7,  8,  9  and  10)  run  in  oil,  in 
an  oil  tight  gear  case  (5). 

Second — The  center  shaft  (3)  supports  the  distributing  cone 
(21)  and  scraping  flights  (20),  while  the  quill  shaft  (12)  sup- 
ports the  rotating  spider  (22). 

Third — The  rotating  screen  frame  (23)  is  cast  in  one  piece  and 
is  removed  by  taking  off  the  hood  (33). 

Fourth — All  bearings  are  of  the  ball  type  and  self -aligning. 

The  operation  is  similar  to  that  of  the  Style  A  machine.  The 
material  enters  at  the  top  (17)  and  receives  the  same  treatment 
as  it  strikes  the  screen,  the  moisture  passing  out  through  open- 


DE-WATERING  OF  FINE  COAL 


281 


ing  (32)  in  each  side  of  the  base  casting,  and  the  solid  material 
falling  down  through  the  passages  (26)  cast  in  each  side  of 
the  base. 

Provision  is  made   in  both  types   of  machine   for  adjusting 
the  distance  between  the  rotating  screen  and  the  scraping  flights. 


Fig.  157.     Cross  Section  Through  Elmore  Dryer  Style  "B" 

CAPACITY,  HORSEPOWER  REQUIREMENTS  AND  OTHER  DATA  FOR  ELMORE 
CONTINUOUS  DRYERS 


Inside 

Type 

diam.  of 
Conical 
Screen 
Frame 
at 

Maximum 
Capacity 
in  cu.  ft. 
per  Hour 

Maximum 
H.P. 
required 

R.P.M. 
Counter- 
shaft 

Size  of 
Driving 
Pulleys 

R.P.M. 
Center 
Spindle 
Shaft 

Net 
Weight 

bottom 

A 

48" 

3500 

35 

525-590 

24  x  10 

400-  450 

19500 

B 

36" 

1500 

20 

450-600 

24  x    6 

600-  800 

10250 

B 

24" 

800 

15 

600-900 

20  x    6 

800-1200 

6000 

TABLE  46 


282 


COAL  WASHING  . 


In  the  Style  A,  this  is  done  by  raising  or  lowering  the  support 
(14).  In  the  Style  B,  the  adjustment  is  made  by  the  screw 
at  the  top  of  the  center  shaft  (3),  the  hub  of  the  rotating  dis- 
tributor (19)  acting  as  the  adjusting  nut. 

The  results  with  the  centrifugal  dryer,  as  far  as  the  delivery 
of  dry  coal  is  concerned,  are  very  satisfactory.  The  moisture 
in  the  dried  coal  is  reduced  to  an  average  of  6  per  cent.  The 


Fig.  158.     Half  Section  and  Front  View  of  Elmore  Dryer  Style  "B" 

power  requirements  are  not  excessive,  dryers  with  a  capacity  of 
60  tons  per  hour  using  from  35  to  50  horsepower. 

The  greatest  disadvantage  noticed  in  the  operation  of  cen- 
trifugal dryers  can  be  traced  to  the  rapid  wearing  of  the  screen 
plates  which,  on  account  of  the  small  perforations,  must  be 
made  of  thin  steel.  A  solution  of  this  problem  would  be  to  use 
a  protecting  grate  inside  of  the  screens  and  to  allow  a  thin 
layer  of  coal  to  remain  on  the  screens.  This  would  act  as  a 


DE-WATERING  OF  FINE  COAL 


283 


filter  bed  and  protect  the  screen  against  the  abrasive  action  of 
the  coal. 

By  partly  dewatering  the  coal  going  into  the  dryer  and 
blanking  the  upper  part  of  the  screen  plates  a  great  improve- 
ment in  the  operation  of  the  dryers  was  noticed,  especially  in 
regard  to  the  fine  coal  retained,  which  before  was  forced  through 
the  perforations.  A  screen  test  of  the  coal  dried  in  an  Elmore 
drier  gave  the  following  results,  showing  that  16  per  cent,  of 
the  coal  leaving  the  dryer  was  finer  than  10  mesh. 


Size  of  dried  washed  Coal 


Percentage 


Held  on  J/&  in.  screen 

Through  J/6  in. — held  on  10  mesh.  .  .  . 
Through  10  mesh — held  on  20  mesh.  . 
Through  20  mesh — held  on  40  mesh.  . 
Through  40  mesh — held  on  60-  mesh .  . 
Through  60  mesh  


70.22 
13.78 
7.80 
4.61 
2.07 
1.52 

100.00 


TABLE  47 


The  idea  of  employing  a  screw  conveyor  on  the  inside  of  a 
centrifuge  and  rotating  this  conveyor  by  means  of  differential 


Fig.  159.     Centrifugal  Coal  Dryer 

gears  at  a  slightly  different  speed  from  the  centrifuge  for  the 
purpose  of  moving  the  coal  continuously  forward,  was  carried 
out  by  Hanrez  (see  page  61 — Fig.  25)  but  without  apparent 


284 


GOAL  WASHING 


success.     A  centrifugal  drier  which  avoids  the  use  of  a  scraper 
inside  of  the  revolving  screen  is  shown  in  Figs.  159  and  160. 

In  a  steel  casing  "a,"  a  conical  screen  "b"  is  located.  The 
arms  "c"  connect  this  screen  with  a  horizontal  shaft  "d," 
which  is  supported  in  two  bearings  ' '  e "  and  is  driven  by  means 
of  the  pulley  "f."  On  this  shaft  a  sleeve  "g"  is  placed.  This 
sleeve  is  supported  on  its  left  end  by  a  sliding  bearing  "h" 
which  also  permits  of  giving  a  slight  eccentricity  to  the  sleeve 
in  regard  to  the  main  shaft  "d."  On  its  right  end  the  sleeve 
fits  over  a  ball  "i"  which  is  fastened  to  the  main  shaft  "d." 
The  sleeve  carries  by  the  arms  "k"  an  annular  plate  "1"  which 


Fig.  160.     Centrifugal  Coal  Dryer.     Showing  Method  of  Operation 

closes  the  smaller  end  of  the  conical  screen,  with  the  exception 
of  a  central  feed  opening  "m."  The  coal  is  fed  to  the  centri- 
fuge by  means  of  a  screw  "n."  The  process  of  dewatering  is 
carried  on  as  follows :  The  washed  coal  is  dewatered  in  an  ele- 
vator to  about  30  per  cent,  of  moisture  and  conveyed  by  the 
screw  "n"  to  the  inside  of  the  centrifuge,  where  it  is  pressed 
against  the  screen.  By  moving  the  bearing  "h"  so  as  to  give 
the  sleeve  an  eccentric  motion,  the  annular  plate  "1"  does  not 
revolve  in  a  plane  at  right  angle  to  the  main  shaft,  but  in  a 
plane:  I-II.  (Fig.  160.)  This  shoves  the  coal  forward,  be- 
cause at  each  revolution  an  annular  wedge  space  I-II-III  is 
cleared  of  coal.  An  amount  of  coal  equal  to  this  space  is  dis- 
charged at  each  revolution  at  "o"  and  simultaneously  replaced 
by  fresh  coal  fed  in  through  the  screw  "n."  The  water  forced 


DE-WATERING  OF  FINE  COAL  285 

out  through  the  perforation  of  the  screen  is  collected  in  the  outer 
casing  and  carried  away  at  "p." 

This  machine  with  a  screen  having  an  average  diameter  of 
4  ft.  10  in.  and  a  length  of  20  in.,  making  300  r.p.m.  dewatered 
30  tons  of  coal  to  7-8  per  cent,  moisture  per  hour.  Washed  coal 
containing  8  per  cent,  of  material  finer  than  160  mesh  was  de- 
watered  at  a  rate  of  28  tons  per  hour.  The  dried  coal  contained 
at  an  average  of  12  per  cent,  of  moisture.  The  power  required 
for  a  capacity  of  28  tons  per  hour  was  16  h.p. 

At  present,  centrifugal  dryers  are  the  most  efficient  pieces  of 
apparatus  we  have  for  the  purpose  of  reducing  the  moisture 
in  the  washed  coal  below  10  per  cent.  It  should  also  be  stated 
that  the  coal  feed  to  the  dryers  must  be  partially  dewatered 
to  at  least  15  per  cent,  moisture,  which  can  be  easily  accom- 
plished by  means  of  a  dewatering  elevator. 

Filtering  apparatus  can  only  be  used  for  fine  coal  and  is  best 
adapted  for  the  dewatering  of  sludge.  Such  devices  will  be 
described  in  connection  with  sludge  recovery. 


CHAPTER  XXIX 
WATER  CLARIFICATION  AND  SLUDGE  RECOVERY 

The  clarification  of  the  wash  water  and  sludge  recovery  are 
carried  on  side  by  side  in  one  process.  The  dirty  wash  water 
is  separated  into  clear  water  on  the  one  hand  and  concentrated 
sludge  on  the  other.  The  clear  water  flows  to  the  pump  cis- 
tern and  from  there  is  put  into  circulation  again  by  pumps. 
The  concentrated  sludge  is  either  mixed  with  the  washed  coal, 
with  or  without  further  treatment,  or  stored  away  in  separate 
bins  for  boiler-house  use;  or  even  in  the  worst  case  wasted  on 
the  refuse  dump.  The  materials  to  be  considered  consist  of 
the  overflow  water  from  the  settling  tanks  and  the  dewatering 
apparatus. 

The  process  of  clarfying  is  carried  on  either  in  large  settling 
basins  or  in  a  series  of  pointed  boxes  (spitzkasten).  The  em- 
ployment of  clearing  basins  has  been  almost  abandoned  for  rea- 
sons previously  given.  The  use  of  spitzkasten  has  never  be- 
come popular  on  account  of  the  large  floor  space  required  and 
the  difficulty  of  removing  the  concentrated  sludge.  In  a  few 
isolated  installations  conical  clearing  tanks  of  large  dimensions, 
similar  to  the  Callow  tanks,  have  been  built  but  the  resulting 
sludge  could  not  be  drawn  off  in  a  sufficiently  concentrated  state 
or  with  any  degree  of  regularity.  The  Dor  thickeners  which 
were  taken  over  from  ore-dressing  plants  have  given  thus  far 
the  most  satisfactory  results. 

The  clarification  of  the  wash  water  must  be  carried  out  to 
such  a  degree,  that  considering  the  necessary  addition  of  fresh 
water  no  increase  in  specific  gravity  shall  occur.  Since  the 
quantity  of  fresh  water  required  to  make  up  for  the  loss  caused 
by  evaporation,  the  water  carried  away  by  the  coal,  refuse  and 
sludge  and  by  leakages,  can  be  easily  determined,  we  can  state: 
The  water  clarification  is  to  be  carried  to  such  a  point  that  the 
addition  of  fresh  water  shall  not  exceed  the  loss  of  wash  water. 

286 


SLUDGE  RECOVERY  287 

This  means  that  no  water  shall  be  wasted  on  account  of  its  being 
too  dirty  to  be  put  back  into  circulation.  The  reason  for  this 
is  that  the  cost  of  water,  on  account  of  the  immense  quantities 
used,  is  quite  a  consideration.  A  washery  treating  2000  tons 
in  eight  hours  circulates  in  that  time  over  1^  million  gallons  of 
water. 

The  cost  of  water  clarification  and  sludge  recovery  should  be 
as  small  as  possible.  Little  has  been  done  in  the  way  of  im- 
provement in  this  direction.  The  apparatus  employed  for  set- 
tling out  the  sludge  should  be  arranged  in  such  a  way  that 
unnecessary  power  requirements  for  the  conveying  of  sludge 
and  water  may  be  avoided.  Two  methods  can  be  used  to  ac- 
complish this:  (1)  The  settling  apparatus  may  be  located  at 
such  an  elevation  that  the  overflow  water  from  the  tanks  can 
flow  by  gravity  to  the  clarifying  apparatus.  This  will,  however, 
require  in  most  cases  a  lifting  of  the  cleared  water  and  the 
concentrated  sludge  to  their  respective  places.  (2)  The  clarify- 
ing apparatus  may  be  placed  sufficiently  high  so  that  the  cleared 
water  as  well  as  the  concentrated  sludge  can  flow  by  gravity  to 
the  places  where  they  are  to  be  used.  In  this  case  the  over- 
flow water  from  the  settling  tank  must  be  lifted  to  the  top  of 
the  clarifying  apparatus.  This  latter  arrangement  has  the  ad- 
vantage that  it  avoids  the  troublesome  elevating  of  the  concen- 
trated sludge  and  furthermore  that  it  makes  the  space  under- 
neath the  clarifying  apparatus  accessible.  The  materials  used 
for  the  construction  of  the  settling  tanks  are  usually  either  tim- 
ber (redwood),  steel  or  reinforced  concrete.  The  concentrated 
sludge  can  be  conveyed  by  means  of  centrifugal  pumps,  dia- 
phragm pumps  or  by  compressed  air.  Centrifugal  pumps  can 
be  used  when  the  sludge  must  be  elevated  above  the  permissible 
height  of  suction.  Diaphragm  pumps  can  only  be  used  on  suc- 
tion lifts  and  are  really  used  more  often  as  a  device  wherewith 
to  regulate  the  flow  of  sludge  than  as  a  conveying  medium. 
Compressed  air  has  been  largely  used  in  Europe  for  conveying 
the  sludge  from  the  clearing  basins.  In  Fig.  161  the  arrange- 
ment of  such  an  installation  is  clearly  shown. 

The  four  discharge  points  of  the  clearing  basin  A  are  con- 
nected by  the  pipes  P  with  the  tank  T.  Communication  be- 
tween any  of  the  four  discharge  points  of  the  clearing  basin 


288 


COAL  WASHING 


Fig.  161.     Apparatus  for  Conveying  Sludge  by  Compressed  Air 


SLUDGE  RECOVERY 


289 


and  the  tank  T  can  be  made  and  interrupted  by  the  valves  V 
located  in  the  pipes  P.  From  the  tank  T  the  pipe  J  leads  to 
the  air  compressor  C.  The  three-way  cock  D  permits  connection 
of  the  tank  T  through  the  pipe  J  either  with  the  atmosphere  or 
with  the  compressor  C.  To  start  operation,  the  pipe  J  is  con- 
nected with  the  atmosphere  and  the  valve  V  is  opened  at  the 
same  time.  This  permits  the  sludge  to  flow  into  the  tank  T. 
Should  the  sludge  not  flow  as  freely  as  desired,  the  cock  D  can 
be  turned  in  such  a  way  that  the  compressor  takes  the  air  from 
the  tank  T,  creating  thereby  a  partial  vacuum  in  the  tank.  This 
accelerates  the  flow  of  the  sludge.  A  float  indicates  the  amount 
of  sludge  in  the  tank.  As  a  further  safeguard  the  pipe  J  is 
carried  well  above  the  top  of  the  clearing  basin,  so  that  no  sludge 
can  enter  the  compressor.  When  the  tank  has  been  filled  with 
sludge,  the  valve  V  is  closed,  the  compressor  started,  delivering 
compressed  air  into  the  tank  through  the  pipe  N.  Now,  by 
opening  the  valve  M  the  sludge  is  forced  out  of  the  tank. 

The  question  yet  remains  as  to  whether  pumps  or  compressed 
air  is  preferable  for  the  conveying  of  sludge.  Conveying  by 
means  of  compressed  air  is  mechanically  more  perfect.  The 
sludge  can  be  thicker  than  if  handled  with  pumps,  without  in- 
creasing the  wear  and  tear  on  the  apparatus.  But  the  cost  of 
the  installation  is  considerably  higher  and  the  operation  re- 
quires  more  careful  attention.  Smaller  washeries  will  there- 
fore prefer  pumps,  especially  if  the  nature  of  the  sludge  is 
such  that  the  wear  and  tear  on  the  pumps  is  not  excessive. 
Larger  washeries  having  great  quantities  of  sludge  to  handle 
should  consider  compressed  air  as  a  medium  for  conveying  it, 
especially  as  an  air-compressing  plant  is  more  or  less  a  necessity 
around  a  mine. 

The  following  table  shows  some  results  obtained  with  spitzkas- 
ten  clearing  basins: 


Capacity            Total 
of               Clearing 
Washer         Surface  of 
per  Hour      Spitzkasten 
in  Tons          in  Sq.  Ft. 

Number 
of  Boxes 

Cleared 
Water 
per 
Minute 
in  Gallons 

Concentrated 
Sludge 

Minute 
in  Gallons 

Power  Required 
to  Lift 
Sludge,           Water, 
Hp.                 Hp. 

100 
150 
200 

860-1620 
1076-2152 
2152-3230 

3-  6 
5-  8 
5-12 

1765-4414 
2647-6621 
3530-8828 

4.4-22 
9.0-33 
17.5-44 

5-15 
6-30 
10-30 

60-  80 
70-100 
90-130 

TABLE  48 


290  COAL  WASHING 

As  mentioned  previously,  the  process  of  clarifying  the  water 
is  carried  on  either  in  large  settling  basins  or  in  a  series  of  spitz- 
kasten,. In  actual  fact,  however,  little  has  been  accomplished  in 
this  respect.  In  most  cases  the  same  water  is  used  over  and 
over  again  until  it  becomes  too  thick  for  any  further  use.  It 
was,  and  still  is,  the  common  practice  to  run  a  washery  with 
one  filling  of  water,  according  to  the  nature  of  the  raw  coal, 
say  for  from  three  days  to  two  weeks,  and  at  the  end  of  this 
period  to  empty  all  the  jig  and  settling  tanks  and  fill  them  up 
again  with  fresh  water.  This  is  a  crude  method,  but  for  the 
lack  of  something  better  it  was  tolerated  even  if  every  washer- 
man condemned  it. 

Fig.  162  shows  the  construction  of  a  settling  tank  with  at- 
tached " spitzkasten."  The  accumulated  sludge  is  removed 
from  the  apexes  of  the  spitzkasten  by  means  of  a  centrifugal 
pump,  which  deposits  this  sludge  either  on  the  washed  coal  or 
carries  it  to  continuous  drum  type  filters. 

The  deplorable  condition  mentioned  above,  namely  the  prac- 
tice of  changing  the  wash  water  rather  than  clarifying  it,  re- 
mained unchanged  until  the  advent  of  the  Dor  thickener.  This 
apparatus  embodies  a  highly  efficient,  economical  and  mechan- 
ically perfect  device  for  settling  out  the  fine  impurities.  The 
Dor  thickeners  make  it  possible  to  recover  as  a  clean,  granular 
coal  material  which  normally  goes  to  waste,  and  at  the  same 
time  furnishes  a  wash  water  as  pure  as  originally  supplied  to 
the  system.  The  operation  of  these  thickeners  is  entirely  au- 
tomatic and  continuous.  Power  and  operating  cost  are  almost 
negligible.  They  may  be  installed  in  any  form  of  circular  tank 
or  basin  up  to  200  ft.  in  diameter.  If  the  nature  of  the  ground 
permits,  simple  excavations  with  concrete  overflow  rims  are  often 
used. 

The  settled  selids  are  continuously  discharged  in  the  under- 
flow as  thick  sludge.  The  operation  of  the  thickener  may  be 
so  controlled  as  to  deliver  an  overflow  either  entirely  clear  or 
containing  a  certain  percentage  of  solids.  For  an  installation  of 
given  size,  the  natural  settling  rate  of  the  material  being  handled 
and  the  rates  of  feed  and  of  underflow  determine  the  amount 
of  solids  in  the  overflow. 


SLUDGE  RECOVERY 


291 


pug 


Section  CC 


292 


COAL  WASHING 


It  has  been  found  that  the  thickener  works  best  if  the  feed 
does  not  contain  material  larger  than  20  mesh.  As  the  over- 
flow from  the  washed-coal  settling  tanks,  and  more  especially 
from  the  centrifugal  dryers,  contains  a  good  deal  of  coal  bigger 
than  20  mesh,  it  is  advisable  to  put  in  a  classifier  ahead  of  the 
thickener  for  the  purpose  of  removing  the  coarse  particles  of 
coal  in  a  dewatered  state  and  to  pass  only  the  fine  slime  to  the 
thickeners. 

The  Dor  classifier,  as  shown  in  Fig.  163,  consists  of  a  shallow, 
rectangular  tank  with  a  sloping  bottom.  Th,e  tank  may  be  set 


Fig.  163.    Dor  Classifier 

at  any  desired  slope,  usually  about  2l/2  in.  to  the  foot.  The  feed 
to  the  classifier  is  continuous;  all  granular  material  settling  to 
the  bottom  of  the  tank  is  raked  up  the  incline  by  reciprocating 
rakes  and  discharged  at  the  high  end  above  the  water  level. 
The  fine  and  more  slowly  settling  solids  overflow  with  the  ex- 
cess water  at  the  opposite  end.  Broadly  speaking,  the  slope  of 
the  bottom,  the  speed  of  the  rakes,  and  the  dilution  of  the  feed 
determine  the  character  of  the  two  products. 

The  classifier  serves  to  dewater  the  granular  coal  and  to  re- 
move the  remaining  small  amounts  of  coal  slime,  which  can  be 
settled  out  in  the  thickeners.  Fig.  165  shows  a  Dor  thickener 
of  70  ft.  diameter  with  concrete  tank.  The  flow  sheet  given  in 
Fig.  164  shows  a  typical  arrangement  for  a  water-clarification 
and  sludge-recovery  plant. 


SLUDGE  RECOVERY 


293 


The  power  required  for  operating  a  70-ft.  Dor  thickener  is 
about  1.5  hp.,  and  the  speed  of  the  rakes  is  approximately  from 
4  to  8  revolutions  per  hour. 

Under  normal  conditions  of  the  overflow  water  from  the 
washed-coal  settling  tank  30  gal.  per  minute  can  be  cleared  per 
100  sq.  ft.  of  settling  area,  so  that  a  70-ft.  thickener  will  be  able 
to  handle  the  overflow  water  from  a  washery  treating  100  tons 
of  coal  per  hour,  if  we  assume  that  the  water  required  for  wash- 


f/NA 


Fig.  164.     Flow  Sheet  for  Water  Clarification  and  Sludge  Recovery  Plant 

ing  wTill  be  three  times  the  weight  of  the  coal,  or  723  gal.  of  water 
per  ton.  The  overflow  can  be  easily  cleaned  so  that  it  does 
not  contain  more  than  2  grams  of  solids  per  liter  (approximately 
117  grains  per  gallon)  or  only  0.2  per  cent,  of  solids.  The  un- 
derflow or  the  sludge  can  be  concentrated  so  that  it  will  contain 
up  to  58  per  cent,  of  solids.  This  is  about  the  limit  of  density 
that  will  still  permit  the  handling  of  the  sludge  through  pipes 
or  with  pumps. 

The  Dor  thickener  mechanism  is  shown  in  Fig.  166  installed 
in  a  steel  tank.     The  tank  may  be  constructed  either  of  steel, 


294 


COAL  WASHING 


wood  or  concrete  as  the  nature  of  the  service  and  comparative 
cost  may  dictate.  The  feed  enters  the  tank  at  the  center  from 
above.  The  solids  settle  to  the  bottom  of  the  tank,  while  the 
liquid  overflows  at  the  periphery  into  a  collecting  trough.  The 
thickener  mechanism,  suspended  in  the  tank  from  the  super- 
structure above,  consists  of  a  central  vertical  shaft  with  radial 
arms  equipped  with  ploughs  to  bring  the  settled  solids,  by  means 
of  a  slow  rotation  of  the  mechanism,  to  a  discharge  opening  at 
the  center  of  the  bottom.  The  settled  solids  as  a  thick  sludge 


Fig.  165.     70  Ft.  Dor  Thickener 


can  be  discharged  at  this  point  by  gravity  or  piped  to  a  pump 
for  delivery  to  any  desired  point. 

The  superstructure  carrying  the  mechanism  may  be  of  steel 
or  wood,  and  may  be  supported  by  the  tank  or  independently. 
If  convenient  the  superstructure  may  be  incorporated  with  the 
roof  trusses  of  the  tank  covering,  should  such  be  provided. 
Power  is  delivered  to  the  mechanism  by  means  of  pulley  and 
worm  reduction  gearing. 

Arrangements  are  provided  for  quickly  and  easily  raising 
the  shaft  and  arms  so  that  they  will  not  become  embedded  in 
the  settled  solids  should  the  power  be  shut  off  for  any  length 


SLUDGE  RECOVERY 


295 


of  time.     The  shaft  can  be  gradually  lowered  again  while  run- 
ning. 

The  operation  of  the  thickener  may  be  so  controlled  as  to  de- 
liver an  overflow  either  entirely  clear  or  containing  a  desired 
percentage  of  solids.  The  proportion  of  liquid  in  the  sludge 
discharge  can  also  be  varied  at  will  between  wide  limits.  For 
a  thickener  of  given  size  the  natural  settling  rate  of  the  solid 
matter  in  the  material  being  handled  and  the  rates  of  feed  and 
underflow  determine  the  amount  of  solids  in  the  overflow. 




"::'*MS£ 


Fig.  166.     Dor  Thickener  Mechanism  Installed  in  Steel  Tank 

The  thickener  may  also  be  operated  to  separate  the  suspended 
solids  into  two  sizes.  Coarse  particles  settle  more  rapidly  than 
fine  ones  of  the  same  specific  gravity,  so  that  it  is  possible,  by 
suitable  adjustment  of  operating  conditions,  to  secure  a  rea- 
sonably clear-cut  separation  of  the  solids  at  a  given  fineness. 
When  so  used  a  thickener  is  termed  a  hydroseparator. 

The  following  results  of  actual  operation  show  the  efficiency 
of  Dor  thickeners  for  bituminous  as  well  as  anthracite  coal. 

Bituminous  Coal.  A  plant  washing  4000  tons  of  raw  coal  in 
16  hr.  for  coking,  has  an  installation  of  four  Dor  thickeners. 
Three  of  these,  70  ft.  in  diameter,  take  the  overflow  from  the 
washed  coal  settling  basin,  and  the  fourth,  50  ft.  in  diameter, 
the  overflow  from  the  refuse  basin. 


296  COAL  WASHING 

The  coal  slush,  amounting  to  6157  gal.  per  minute,  contains 
1.25  per  cent,  solids,  or  275  tons  per  day  of  coal.  Of  this  total 
97.87  per  cent.,  or  269  tons,  is  recovered  in  the  form  of  a  sludge 
carrying  48  per  cent,  moisture.  This  sludge  is  delivered  on  top 
of  the  coarser  coal  on  the  washed  coal  belt. 

The  overflow  from  the  larger  thickeners,  carrying  less  than 
0.15  per  cent,  solids,  together  with  the  overflow  from  the  50  ft. 
refuse  thickener,  is  returned  to  the  washery  circulation.  This 
return  constitutes  98  to  99  per  cent,  of  the  water  in  the  feed  to 
the  thickener. 

Exclusive  of  plant  leakages,  the  net  consumption  of  water  is 
thus  reduced  to  approximately  40  gal.  per  ton  of  raw  coal  washed 
or  the  amount  carried  away  in  the  washed  product,  and  the 
refuse. 

Anthracite  Coal.  A  typical  Dor  recovery  plant  is  handling 
the  slush  from  an  anthracite  breaker  producing  5000  tons  per 
day  of  8  hr.  The  slush  is  made  through  a  %4  in.  round  hole 
screen,  the  quantity  varying  from  3000  to  4500  gal.  per  min., 
the  average  ratio  of  water  to  solids  being  about  30  to  1.  The 
solids  average  45  long  tons  per  hour. 

The  plant  consists  of: — One  Dor  hydroseparator  26  ft.  in 
diameter  by  7  ft.  deep,  the  arms  of  which  revolve  at  1  r.p.m., 
three  Model  "C"  duplex  Dor  classifiers,  5  ft.  6  in.  wide  by 
18  ft.  long,  speed  16  strokes  per  minute,  slope  2a/4  in.  per  foot; 
and  a  scraper  line  for  removal  of  the  finished  product. 

The  slush  is  delivered  at  the  center  feed  well  of  the  hydro- 
separator.  The  overflow  from  the  separator,  consisting  of  the 
greater  part  of  the  water  and  extremely  fine  material  is  run 
to  waste.  The  underflow  carrying  all  the  coarser  granular  ma- 
terial gravitates  to  the  Dor  classifiers  where  the  final  separa- 
tion and  dewatering  takes  place. 

Apart  from  the  scraper  line  for  the  removal  of  the  finished 
product;  the  entire  recovery  plant  requires  5  to  6  h.p.  Attend- 
ance requires  only  a  small  fraction  of  one  man's  time. 

In  cases  where  the  slush  contains  a  large  percentage  of  ash, 
this  ash  can  be  effectively  reduced  at  a  small  cost  by  the  use 
of  concentrator  tables.  Where  this  is  done,  such  tables  are  ii 
stalled  between  the  hydroseparator  and  the  classifiers, 


SLUDGE  RECOVERY  297 

The  attached  skeleton  flow  sheet  gives  typical  results  on  anth- 
racite slush,  both  with  and  without  concentrator  tables. 

In  either  case  the  final  coal  product  (classifier  discharge)  is 
discharged  at  approximately  30  per  cent,  moisture.  Either  in 
cars  or  stock  pile,  it  drains  rapidly  to  12  to  15  per  cent,  moisture. 

It  should  be  pointed  out  that  the  procedure  in  any  given 
case  depends  upon  the  use  to  which  the  recovered  fuel  is  to  be 
put.  If  it  is  to  be  mixed  with  other  steam  sizes  and  used  for 
fuel  at  the  mine  boiler  plant  adjustments  are  made  to  give  a 
separation  at  60  to  100  mesh,  as  it  has  been  found  that  coal 
finer  than  this  introduces  difficulty  in  keeping  the  boilers  up 
to  the  desired  rating.  Within  reasonable  limits  the  ash  content 
is  of  secondary  importance. 

If  the  recovered  fuel  is  to  be  used  for  briquetting,  ash  should 
not  exceed  16  to  18  per  cent,  (which  in  most  cases  involves 
tabling)  but  there  are  no  limitations  as  regards  fineness.  If  the 
recovered  fuel  is  to  be  pulverized  and  used  in  this  form  for 
firing,  there  are,  of  course,  no  limitations  as  to  fineness,  and  a 
low  ash  content  is  not  essential.  The  chief  objection  to  high 
ash  in  coal  used  for  this  purpose  is  the  consequent  reduction  of 
the  fuel  value  of  this  coal  and  the  increased  cost  of  pulverizing. 
In  all  but  the  first  case,  the  fuel  must  be  further  dried. 

The  flow  sheet  given  below,  shows  the  results  in  the  recovery 
of  mine  fuel.  The  recovery  normally  amounts  to  25  to  40  per 
cent,  of  the  mine  fuel  consumption.  In  tonnage,  the  mine  fuel 
consumption  amounts,  in  the  anthracite  field,  to  approximately 
10.5  per  cent,  of  the  production. 

In  the  recovery  of  slush  coal  by  the  method  outlined,  the 
requirements  in  each  of  the  three  cases  cited  may  be  met  by 
suitably  adjusting  the  size  of  equipment  installed,  to  the  re- 
sult desired.  Operating  adjustments  also  permit  of  varying 
considerably  the  character  of  the  product. 

Broadly  speaking,  slush  recovery  plants  may  be  so  designed 
and  operated  as  to  produce  any  of  the  following  results: 

I     a — All  the  slush  solids  in  the  form  of  a  sludge  carrying  40  to  45  per 

cent,  moisture. 

6 — Substantially  clear  water  amounting  to  80  to  85  per  cent,  of  the 
water  in  the  slush. 


298 


COAL  WASHING 


II     a — All  the  granular  solids  (separation  at  60  mesh  or  finer  as  desired) 

practically  free  from  slime  and  silt.     This  product  will  stack. 
5 — A  waste  product  carrying  the  balance  of  the  slush  solids  and  the 

bulk  of  the  water. 

Ill  Either  I  or  II  with  the  ash  in  the  recovered  coal  reduced  by  tabling, 

to  a  percentage  approaching  that  of  "stove"  and  "chestnut"  coal. 

In  all  cases  80  to  85  per  cent,  of  the  slush  water  may  be  re- 
covered in  a  substantially  clear  condition,  available  for  re-use 
in  the  breaker. 

Anthracite 
Breaker  Slush 


Mesh 

+  60 
+  100 
+  200 


Cum. 
44.5 


(Ash        38.0  *) 
57.6  t 
721   (Fyrite    2. 1C*) 


Discharge 
G1.5H 


I Hydroseparatorl 
I 


Overflow 
38-  5  * 


Mesh 


Cum. 


+  200  97.39   (Pyrite     2.34* 


••-,1* 

31.71(Pyrite 


£ 

"blel 

Refuse 
19.2* 
Mesh      jCCum. 

CoVl 
42.3* 

*Cum. 

+  00         G5.76   fAsh 
+  100        88.91   r 
+  200         98.42  (Pynte 

51.02*5 
5.84*  ) 

74.42  (Asa     11.75*) 
(Pyrite  0.75i) 

[Dorr  Clcissitierl 


[Dorr  Classifier} 


| 

Overflow 


*0um. 


+  200        49.55 


0.00 

22.63'  (Ash 

)   26.70( 

f  Pvrtta     9  ^7  \  (Py"te    1.66  ) 

99.21 (Pyrlte   2>37)  81.00 


Note  .  Screen  analyses  refer  to  dry  solids 

Figures  underlined  give  percentages 
by  weight  of  solids  In  orginal-slush 
feed. 

Flow  Sheet  of  Dor  Installation  Treating  Anthracite  Breaker  Slush 

J.  R.  Campbell  in  his  paper  on  "The  Mechanical  Separation 
of  Sulfur  Minerals  from  Coal"  gives  the  following  results  from 
two  70  ft.  Dor  thickeners: 


SLUDGE  RECOVERY  299 

The  use  of  the  Dor  thickeners  has  been  very  successful  though  we  ex- 
perienced some  trouble  at  first  due  to  inexperience.  The  following  is  a 
typical  operation  of  these  thickeners  under  normal  conditions: 


Influent 

Effluent 

Underflow 

Welter  per  cent 

98 

99.7 

47.2 

Solids,  coal,  per  cent  
Specific  °Tavity 

2.0 
1  0052 

0.3 

1  0008 

52.8 
1.1580 

Total,  per  cent  
Tons  per  hour 

100 
500 

97 

485 

3 

15 

TABLE  49 

The  above  is  based  on  the  operation  of  two  70-ft.  Dor  tanks  handling 
the  wash  water  from  approximately  1200  tons  of  coal  washed  in  8  hr.  The 
overflow  water  contains  but  a  small  percentage  of  solids  and  is  in  fine  con- 
dition for  reuse.  The  underflow,  or  sludge,  is  in  good  condition  for  handling 
in  a  number  of  ways  and,  from,  an  analytic  standpoint,  takes  on  the  charac- 
ter of  the  washed  coal  proper. 

The  following  are  possible  solutions  of  the  sludge  problem:  It  may  be 
pumped  by  means  of  the  diaphragm  pump  direct  to  the  washed-coal  elevator 
on  top  of  the  coal  in  the  dewatering  buckets  and  passed  through  the  me- 
chanical dryers  with  the  coarse  coal,  or  it  may  be  pumped  direct  to  the 
dryer,  but  this  practice  builds  up  the  circulating  system  and  is  almost  cer- 
tain to  cause  trouble  sooner  or  later. 

The  second  way  is  to  operate  in  an  open  circuit  and  pump  the  50-50 
sludge  direct  on  top  of  the  washed  and  dried  coarse  coal,  which  eliminates 
it  from  the  system  altogether,  although  this  practice  adds  about  3  per  cent, 
moisture  to  the  final  washed  product. 

(3)  The   third,   and   perhaps   the  most  logical  method,   is  to  take  the 
sludge  from  the  Dor  tanks  and  put  it  through  a  continuous  filter  of  ap- 
proved type  and  dehydrate  it  to  12  per  cent,  water  and  under,  after  which 
the  cake  can  be  added  to  the  dried  coarse  coal.     This  method  would  give 
a  final  washed  product  of  minimum  water  content  and  would  add  less  than 
Ic.  per  ton  to  the  cost  of  the  washed  product. 

(4)  A  fourth  way  would  be  to  take  the  cake  from  the  continuous  filter 
and  completely  dehydrate  it  in  a  direct  heat  drier,  adding  the  powder  to 
the  washed  and  dried   coal.     This  method  would  be  more  expensive  and 
seems  of  doubtful  value. 

In  connection  with  the  sludge  recovery  J.  R.  Campbell  de- 
veloped a  formula  for  determining  the  percentage  of  solids  in 
the  influent,  effluent,  and  underflow  from  the  specific  gravity 
of  the  various  solutions,  which  seemed  the  most  expeditious  way 
of  reaching  conclusions  rapidly  without  the  tedious  and  labora- 
tory way  of  filtering  and  weighing  the  solids,  though  this  process 
should  be  followed  at  frequent  intervals  as  a  check. 


300 


COAL  WASHING 


Storage  Reservoir 


Beplenlib  ng  Water 


,ter— -jl 


Elevated  Tank 


Refuse 


&  Water 


Refuse  Settling  Tank 


Wash 


50  Ft.Dorr  Thickener 


Befus*  It 


Water 


d  Coal 


Washed  Coal  Settling  Tank 


Elevators 


,  Ove 

Water 


&  Coal 


WasheJcoal    ||        2-70  Ft.Dorr  Thickeners 


|     Loading  Conveyor 


Coal 


ludge- 


Overflow 
Wttteir 


R.R. 


Cars    \ 


Fig.  167.     Flow  Sheet  for  a  Water  Clarification  and  Sludge  Recovery 

System 


Let  A  =  specific  gravity  of  water,  B  —  specific  gravity  of 
coal  (say  1.35),  and  X  =  specific  gravity  of  solution  in  question; 
then, 


B(X-A) 


X(B-A) 


X  100  =  per  cent,  solids  in  solution. 


To  illustrate,  we  will  use  the  test  on  Dor  thickeners,  the  un- 
derflow having  a  specific  gravity  of  1.158. 


KLUDGE  RECOVERY  301 

(1.158  -  1.000)  X  1.350 


(1.350-1.000)  X  1.158 


X  100  =  52.6  per  cent,  solids. 


The  weighed  percentage  of  solids  in  the  underflow  was  52.8  per 
cent.  In  a  similar  way  the  amount  of  influent,  effluent,  and 
underflow  of  the  Dor  tanks  can  be  determined,  given  any  one 
of  the  three  quantities  from  the  specific  gravities  of  the  solu- 
tions and  consequently  the  water  consumption  of  the  plant,  if 
no  meter  provisions  are  made.  As  it  is  comparatively  easy  to 
measure  the  underflow,  the  calculation  is  usually  made  with  this 
known  quantity  as  to  the  amount  in  a  given  length  of  time. 


CHAPTER  XXX 
SUBSEQUENT  TREATMENT  OF  SLUDGE 

A  sludge  containing  too  much  impurity  to  be  mixed  in  with 
the  washed  coal  entails  great  losses  upon  the  economic  operation 
of  a  washery.  Furthermore,  this  sludge,  if  wasted  upon  the 
refuse  dump,  will  fire  in  course  of  time  and  is  liable  to  cause 
thereby  much  trouble  and  damage. 

The  loss  of  combustible  with  the  sludge  is  of  greater  import- 
ance with  coking  coal,  where  the  fines  are  of  greater  value  than 
with  fuel  coal.  Therefore,  efforts  to  treat  the  sludge  for  fine- 
coal  recovery  are  advisable.  Many  different  methods  have  been 
tried,  but  thus  far  the  results  obtained  have  been  only  mediocre. 
This  is  not  surprising,  considering  the  fineness  of  the  material. 
The  possibility,  however,  of  a  separation  can  be  based  upon  the 
fact  that  even  the  smallest  particles  of  coal  show  a  granular 
structure,  whereas  the  fireclay  or  the  crushed  slate  are  of  such 
a  fineness  that  the  particles  are  held  in  suspension  in  the  water. 

Successful  separation  of  coal  from  the  sludge  demands  a  dis- 
tinct difference  in  the  size  of  the  grains.  The  requirements  are 
that  the  fireclay  shall  be  removed  from  the  sludge  as  much  as 
possible  without  great  loss  of  coal.  Up  to  the  present  time  the 
only  successful  method  for  such  a  separation  depends  upon  a 
swift  current  of  fresh  water  in  the  shape  of  sprays,  but  the  ten- 
dency at  present  leans  toward  the  use  of  apparatus  now  em- 
ployed in  ore-dressing  plants,  such  as  slime  tables  or  Dor 
classifiers. 

One  important  piece  of  apparatus  at  present  operating  at  least 
halfway  successfully  is  the  Kohl-Simon  screen,  shown  in  Fig. 
168.  The  screens  having  fine  brass-wire  mesh  (65  mesh  to  the 
inch)  are  hung  at  their  upper  ends  on  the  swinging  rods  A 
and  on  their  lower  ends  on  the  bails  B.  The  eccentrics  C  give 
the  screens  a  reciprocating  motion  and  at  the  same  time  the 
double  cams  D  impart  to  the  screens  a  forcible  vibrating  motion. 

302 


TREATMENT  OF  KLUDGE 


303 


The  sludge  to  be  treated  is  sluiced  onto  the  screens  through 
the  launder  E.  Fresh-water  sprays  are  forced  against  the  sludge 
through  the  pipe  F,  which  has  Vs-iu.  holes  over  its  whole  length 
on  the  under  side.  These  sprays  wash  the  fireclay,  which  has 
finer  grains  than  the  coal,  through  the  screens  into  the  launder 
G.  The  fine  coal  freed  from  the  fireclay  travels  over  the  screens 


c  H  c 

Fig.  168.     Kohl-Simon  Screen  for  Treatment  of  Sludge 

and  is  collected  together  with  part  of  the  wash  water  in  the  laun- 
der H.  The  following  results  were  obtained  with  this  appara- 
tus : 


Daily 

Input 
in 
Gallons 

Fresh             Clean  Coal  Produced             Resulting    Dirtv    Water 
Water                         Ash       Moisture     Amount            Solid  Matter 
.-f61           Used  in        Tons         Per            Per         of  Water    ,      Ash           Coal 
'nt-         Gallons                       Cent.         Cent.       in  Gallons  Per  Cent.  Per  Cent. 

60,000 

10.39       63,000       4.78       8.16       14.78       120,000       39.74       60.76 

TABLE  50 

Instead  of  shaking  screens,  revolving  screens  are  also  used. 
These  screens  have  a  perforated  zinc  mantel  with  a  fine  brass-wire 
mesh  fastened  securely  thereto  on  the  inside.  The  fireclay  is 
washed  through  the  screen  by  fresh-water  sprays,  just  as  with 
the  shaking  screens. 


304  COAL  WASHING 

The  use  of  slime  tables  is  still  in  an  experimental  state,  but 
judging  from  the  results  obtained  in  the  ore-dressing  plants  a 
successful  operation  can  be  expected.  The  Dor  classifier  has 
been  used  in  the  anthracite  region  to  recover  coal  from  the 
breaker  slush.  Over  55  per  cent,  of  the  coal  contained  in  the 
slush  was  recovered  and  the  ash  reduced  from  30  per  cent,  to 
22  per  cent.  This  was  further  reduced  to  16  per  cent,  by  treat- 
ing the  recovered  coal  on  tables. 

All  known  methods  of  treating  the  sludge  can  only  be  used 
to  a  limited  extent.  Success  can  only  be  expected  if  the  impuri- 
ties are  in  finer  grains  than  the  coal.  This  requires  prelimi- 
nary investigations,  which  will  also  give  data  in  regard  to  the 
size  of  the  screen  perforation.  Sludge  with  30  to  40  per  cent, 
ash  treated  over  screens  with  sprays  gave  a  recovery  of  about 
20  to  30  per  cent,  of  coal  with  from  8  to  10  per  cent,  of  ash. 


DRYING  OF  THE  SLUDGE 


The  sludge,  treated  or  untreated,  must  in  every  case  be  de- 
watered  before  it  can  be  mixed  with  the  washed  coal.  On  ac- 
count of  the  fineness  of  the  material  centrifugal  dryers  cannot 
be  taken  into  consideration.  Heat  dryers  are  not  an  economical 
proposition  and  therefore  we  must  have  recourse  to  filters.  The 
requirements  for  filters  are  identical  with  the  requirements  for 
all  the  other  apparatus  used  in  a  washery — that  is,  continuous 
operation,  high  efficiency,  simplicity  of  construction,  low  cost 
of  installation  and  operation,  and  durability. 

Nobody  will  expect  that  any  one  piece  of  apparatus  will  fulfill 
all  of  the  foregoing  requirements,  but  in  regard  to  filters  the 
continuous  drum-type  comes  nearer  to  doing  it  than  any  other. 
Sludge  containing  35  per  cent,  solids  and  65  per  cent,  liquid 
has  been  dewatered  with  it  to  only  20  per  cent,  moisture.  This 
will  make  it  appear  feasible  that  a  sludge  with  56  per  cent,  solids 
and  only  44  per  cent,  liquids  can  be  brought  down  to  at  least 
10  to  12  per  cent,  moisture.  This  would  put  the  sludge  in  such 
shape  that  it  could  be  mixed  with  the  washed  dried  coal  without 
increasing  the  moisture  content  of  the  final  product  to  any 
appreciable  extent. 

J.  R.  Campbell  is  of  the  opinion 


TREATMENT  OF  SLUDGE  305 

"that  the  most  logical  method  of  solving  the  sludge  problem  would  be  to 
take  the  sludge  from  the  Dorr  thickeners  and  put  it  through  a  continuous 
filter  of  approved  design  and  dehydrate  it  to  12  per  cent,  moisture  and 
under;  after  which  the  dried  sludge  cake  can  be  added  to  the  dried  coarse 
coal.  This  method  would  give  a  final  washed  product  of  minimum  water 
contents  and  add  less  than  one  cent  per  ton  to  the  cost  of  the  washed 
product." 

G.  W.  Evans,  coal  mining  engineer  of  the  Northwest  Experi- 
ment Station,  U.  S.  Bureau  of  Mines,  Seattle,  stated  at  the 
British  Columbia  International  Mining  Convention  that 

"A  coal-cleaning  plant  operating  along  most  modern  lines  does  not  waste 
very  much  except  the  color  in  the  water.  Probably  some  enterprising  engi- 
neer will  attempt  to  recover  the  color  by  means  of  an  Oliver  filter." 

The  purpose  of  a  filter  is  however  not  only  to  remove  the 
color  in  the  water,  but  also  to  put  the  sludge  in  such  shape  that 
it  can  be  added  to  the  dried  coarse  coal. 

Filtration.  For  convenience  in  discussion  we  can  classify  most 
of  the  filtering  problems  into  three  groups. 

First:  Those  in  which  the  suspensions  are  extremely  fine 
or  of  a  flocculent  or  collodial  nature,  causing  a  slow  separation 
of  the  solid  suspensions  from  the  liquid. 

Second :  Those  in  which  the  suspensions  separate  themselves 
readily  from  the  liquid  content,  and  form  a  filter  cake  of  sub- 
stantial thickness  in  a  short  period  of  time. 

Third:  Those  in  which  the  suspensions  are  of  such  a  coarse 
granular  nature  that  the  liquid  content  can  be  readily  with- 
drawn but  the  solids  are  so  coarse  and  of  such  a  high  specific 
gravity  that  they  fail  to  cohere  with  one  another  to  form  a  com- 
pact filter  bed. 

In  the  first  group  the  open  tank  filter  is  generally  the  best 
type  of  apparatus  to  employ. 

In  the  second  group  the  rotary  filter  will  give  the  most  satis- 
factory results. 

In  the  third  group  the  rotary  hopper  dewaterer  is  best  adapted 
to  the  problem. 

The  rotary  filter  made  its  appearance  in  the  latter  half  of 
the  19th  century  in  Belgium,  where  the  idea  of  a  rotating  drum 
with  a  perforated  surface  covered  over  with  filter  medium  was 
conceived.  It  remained  for  George  Moore,  metallurgist,  how- 


306 


COAL  WASHING 


ever,  to  invent  the  multiple  compartment  rotary  filter,  United 
States  Patent  No.  746552.  The  invention  of  the  multiple  com- 
partment rotary  filter  by  Moore  followed  closely  on  the  heels  of 
the  Moore  process,  well  known  in  metallurgical  lines. 

The  multiple  compartment  rotary  filter  is  a  natural  outgrowth 
of  modern  business  efficiency  methods.  The  tendency  of  the 
present  day  is  to  eliminate  intermittent  processes  and  to  replace 
them  with  continuous  and,  if  possible,  automatic  operation.  A 
continuous  method  means  maximum  output  at  the  most  econom- 


Fig.  169.     Portland  Filter 

ical  rate,  and  automatic  machinery  means  the  standardization 
product  and  relief  from  the  uncertain  labor  market. 

At  present  there  are  four  types  of  continuous  filters  on  the 
market,  namely,  the  Portland,  the  Zenith,  the  Oliver,  and  the 
American.  None  of  these  machines  have  been  installed  in  a 
commercial  washery  but  sufficient  laboratory  tests  have  been 
made  to  show  their  capacity  and  efficiency. 

The  Portland  filter  shown  in  Fig.  169  is  of  the  rotary  drum 
type.  It  consists  of  a  series  of  panels  carrying  a  porous  filter 
medium,  arranged  in  the  form  of  a  drum  which  is  adapted  to 


TREATMENT  OF  SLUDGE  307 

rotate  in  a  tank  containing  the  material  to  be  filtered.  Each  of 
the  panels  or  sections  which  make  up  the  drum  is  entirely  inde- 
pendent of  the  others,  and  its  action  is  controlled  by  an  auto- 
matic valve  which  serves  the  entire  machine. 

This  valve  is  one  of  the  distinctive  features  of  the  machine 
and  permits  a  wide  variation  in  the  working  of  the  filter  accord- 
ing to  the  class  of  material  being-  handled  and  the  precise  re- 
quirements of  the  method  of  operation. 

The  panels  are  made  up  of  heavy  redwood  planking,  and  cast 
iron,  or  other  suitable  material,  which  forms  an  impervious  back 
to  the  sections.  Drainage  channels  are  provided  on  the  face  of 
the  panel,  and  upon  these  the  filter  medium  consisting  of  a  layer 
of  rolled  wire  cloth,  a  layer  of  burlap,  and  a  final  surface  of  a 
cotton  fabric  of  proper  texture,  are  successively  placed.  In  ex- 
ceptional cases  the  filtering  medium  consists  of  a  special  woven 
wire  cloth  of  non-corrosive  metal,  which  serves  the  same  purpose 
as  the  cotton  fabric.  While  the  expense  of  manufacture  is  great, 
the  initial  cost  of  a  filter  covering  of  this  kind  is  under  certain 
conditions  more  than  counterbalanced  by  its  correspondingly 
long  life. 

The  entire  drum  is  wound  helically  with  wire,  leaving  the  filter 
surface  exposed  between  the  convolutions,  which  are  about  one- 
half  inch  apart.  The  wire  cloth  is  a  permanent  part  of  the 
panel,  only  the  burlap  and  drill,  together  with  the  winding  of 
wire  requiring  renewal,  and  these  only  at  long  intervals. 

The  lower  part  of  the  drum  is  submerged  in  a  tank  containing 
the  material  to  be  filtered  and  is  slowly  rotated — one  revolution 
in  five  to  eight  minutes — taking  on  a  layer  or  cake  of  solids 
which  is  discharged  before  the  same  portion  of  the  filter  surface 
again  enters  the  pulp.  To  build  up  the  cake  and  remove  the 
clear  liquid  a  vacuum  is  maintained,  and  to  assist  in  discharging 
the  cake  and  cleanse  the  filter  medium  a  pressure  of  air  is  ad- 
mitted to  the  interior  of  each  panel  at  the  proper  point  in  the 
cycle  of  revolution. 

A  scraper  maintained  in  contact  with  the  working  face  of  the 
drum,  just  above  the  top  of  the  tank  on  the  descending  side,  re- 
ceives the  cake  as  it  is  dislodged  by  the  air  and  deflects  it  outside 
the  tank  for  disposal. 

The  Zenith  rotary  filter  is  shown  in  Fig.  170.     It  consists  of 


308 


COAL  WASHING 


a  hollow  drum  mounted  upon  a  horizontal  axis,  the  lower  por- 
tion of  the  rotating  drum  dipping  into  the  container  holding  the 
slurry  to  be  filtered.  The  outer  surface  of  the  filter  drum  is  di- 
vided into  a  number  of  uniform  shallow  compartments,  each 
compartment  being  connected  by  separate  pipe  lines  to  the  cen- 
tral valve  hub  which  is  cored  out  to  receive  the  pipes  from  each 
compartment.  The  compartments  are  covered  with  wire  screen 
suitably  supported  and  over  this,  encircling  the  entire  drum  is 
stretched  the  filtering  medium,  each  compartment  being  kept 


Fig.  170.     Zenith  Filter 

separate  and  distinct  and  giving  a  smooth  peripheral  surface  to 
the  drum. 

The  central  valve  hub  rotates  against  a  stationary  valve  cap 
which  is  so  arranged  that  each  compartment  can  independently 
be  subjected  to  suction,  or  neutral,  or  pressure  during  any  por- 
tion of  the  cycle.  Provision  is  made  in  the  valve  cap  for  apply- 
ing suction  and  for  drawing  off  the  filtrate  and  wash  water  sepa- 
rately if  desired. 

The  container  in  which  the  drum  rotates  is  hopper  bottomed 
and  provided  with  a  mechanical  agitator  for  keeping  the  slurry 
in  a  constant  state  of  agitation  during  filtration.  A  scraper  is 
fitted  to  the  edge  of  the  container,  parallel  to  the  axis  and  set  at 
an  extremely  short  distance  from  the  face  of  the  drum.  It  is 


TREATMENT  OF  SLUDGE  309 

the  purpose  of  the  scraper  to  remove  the  cake  from  the  surface 
of  the  drum  just  before  that  portion  of  the  drum  is  about  to  dip 
into  the  solution.  Washing  of  the  cake  is  accomplished  by 
sprays  which  are  so  placed  as  "to  play  upon  the  surface  of  the 
drum  just  after  the  drum  with  its  accumulated  solids  emerges 
from  the  container. 

The  solids,  held  upon  the  filtering  surface  by  suction,  pass  out 
of  the  slurry,  are  washed  by  the  sprays  and  discharged  over  the 
scraper  as  a  dry  cake  in  the  form  of  a  ribbon  from  whence  they 
may  be  conveyed  as  desired. 

The  surface  of  the  filtering  medium  is  protected  from  abrasion 
by  wire  which  is  wound  around  the  drum  in  the  form  of  a  helix. 
All  parts  of  the  machine  are  sturdily  constructed;  the  castings 
are  perfect,  the  wearing  surfaces  nicely  machined  and  ground. 
The  drum  is  driven  by  a  large  well  cut  gear  and  worm.  All  the 
parts  of  this  machine  are  made  for  rough  service  and  long  use 
and  for  this  reason  the  items  of  repair  and  renewal  are  low. 

Every  part  of  the  filter  is  easily  accessible.  The  pipes  are 
extra  large  and  the  valve  sector  shaped  so  as  to  readily  take  care 
of  the  large  amounts  of  free  air  and  filtrate  which  are  drawn 
through  the  filtering  medium  thus  giving  a  maximum  efficiency 
per  square  foot  of  active  filtering  surface  and  permitting  of  no 
reduction  in  vacuum  between  valve  and  filtering  medium.  This 
fact  is  of  special  importance  and  is  a  distinctive  feature  of  the 
Zenith  rotary  filter. 

The  drum  usually  is  rotated,  at  a  rate  of  about  one  revolution 
in  five  minutes.  The  container  is  kept  filled  with  the  slurry  to 
be  filtered,  the  excess  being  taken  care  of  by  an  overflow  pipe, 
and  the  solution  is  kept  in  agitation  by  means  of  the  mechanical 
agitator.  Vacuum  is  maintained  by  a  dry  vacuum  pump  or 
other  means  of  creating  suction  of  ample  capacity  to  take  care 
of  the  filtrate  plus  the  free  air  which  is  drawn  through  the  filter- 
ing medium.  When  operated  with  a  dry  vacuum  pump,  a  tank 
acting  as  a  receiver  for  the  filtrate,  is  interposed  between  the 
pump  and  the  filter.  This  tank  is  first  exhausted  by  the  pump 
and  suction  to  the  filter  is  applied  from  here.  The  suction  causes 
the  clear  filtrate  to  be  drawn  through  the  filtering  medium,  large 
pipes  and  valve  to  the  receiver. 

From  here  the  filtrate  may  be  removed  continuously  and  auto- 


310  COAL  WASHING 

matieally  by  means  of  a  centrifugal  pump  or  intermittently  by 
cross  connecting  two  receivers. 

The  solids  are  drawn  by  the  suction  on  to  the  face  of  the  drum 
forming  a  uniform  layer  or  cake.  As  the  drum  rotates  this  layer 
or  cake  adhering  to  the  surface  of  the  filtering  medium  emerges 
from  the  solution  and  suction  being  maintained,  the  mother 
liquor  is  drawn  out  of  the  cake. 

When  about  to  reach  the  topmost  point  of  rotation  (should 
washing  be  desired)  the  cake  is  subjected  to  wash  water  delivered 
from  sprays  arranged  a  few  inches  above  the  surface  of  the  drum. 
A  sheet  of  water  is  thus  spread  over  the  cake  so  regulated  that 
no  water  runs -back  into  the  container.  The  suction  causes  this 
water  to  be  drawn  through  the  cake  replacing  the  mother  liquid 
held  in  its  interstices,  thus  giving  a  most  excellent  wash.  Should 
it  be  desired  to  keep  the  wash  water  separate  from  the  original 
filtrate,  the  former  may  be  led  off  separately  through  the  specially 
designed  valve  to  the  proper  receiving  tank. 

After  passing  the  zone  of  sprays  the  cake  is  air  dried  before 
being  discharged  over  the  scraper.  Just  before  reaching  the 
scraper,  however,  the  suction  is  automatically  cut  off  from  that 
particular  compartment.  At  times  it  is  advisable  to  apply  a 
blast  of  air  to  the  compartment  at  the  point  of  discharge  in  order 
to  open  the  pores  of  the  filtering  medium  while  the  cake  is  being 
removed,  thus  leaving  a  clean  medium  with  which  to  begin  a  new 
cycle.  As  the  cake  is  discharged  over  the  scraper  in  the  form 
of  a  ribbon,  it  may  be  dumped  into  a  receptacle  and  carried  away 
at  intervals  or  it  may  be  fed  onto  an  automatic  conveyor  and 
thus  collected  continuously. 

The  slurry  in  the  filter  container  is  kept  in  uniform  agitation 
by  means  of  a  two  blade  agitator  in  the  bottom  of  the  container. 
These  agitators  are  connected  to  the  driving  mechanism  of  the 
machine  and  require  no  separate  connection  or  attention. 

The  machine  is  belt  driven,  with  a  heavy  worm  and  gear  con- 
nection to  the  drums  on  the  opposite  side  from  the  vacuum  line 
so  that  a  steady  even  turning  movement  is  maintained  with  little 
friction  or  wear  upon  the  gears. 

Should  an  especially  dry  cake  be  desired,  or  should  a  cake 
crack  badly,  the  filter  may  be  fitted  with  jacks  or  pressure  shoes 
which'  subject  the  cake  to  compression  after  leaving  the  sprays. 


TREATMENT  OF  SLUDGE  311 

Under  such  conditions  the  cracks  are  ironed  out  and  the  cake  is 
under  both  suction  and  pressure,  thus  insuring  an  exceptionally 
dry  product. 

It  is  thus  seen  that  during  every  revolution  of  the  filter  drum 
the  solids  are  picked  up,  washed,  dried  and  carried  away  con- 
tinuously and  automatically,  no  labor  being  required.  The  prod- 
ucts are  washed  and  discharged  with  no  uncertainty  and  the  ma- 
chine registers  a  large  capacity  for  the  space  occupied. 

The  rotary  filter  is  made  in  many  sizes  ranging  from  a  labora- 
tory unit  having  a  drum  1  ft.  in  diameter  with  a  6  in.  face,  up  to 
a  unit  8  ft.  in  diameter  with  an  8  ft.  face.  The  filter  is  con- 
structed with  either  a  high  or  low  container  according  to  the 
properties  of  the  solution  to  be  filtered.  With  the  high  container 
the  sides  rise  well  above  the  hub  of  the  drum,  and  with  the  low 
container  the  upper  edge  is  just  below  the  hub. 

The  high  container  is  used  for  those  solutions  where  the  period 
of  loading  or  filtering  should  be  long  compared  with  the  period  of 
washing,  drying  and  discharge.  The  low  container  is  for  the 
reverse  condition  or  where  there  is  an  exceedingly  rapid  filtering 
rate. 

The  drum  of  the  filter  may  be  constructed  with  enclosed  ends 
for  protecting  the  pipes  leading  from  the  compartments  to  the 
hub,  should  this  be  necessary. 

The  filter  drum  of  wood  and  iron  is  supported  by  heavy  cast 
iron  spiders  across  which  are  bolted  the  thick  wooden  staves 
forming  the  drum  proper. 

The  filtering  medium  is  furnished  according  to  the  require- 
ments of  the  slurry  to  be  filtered.  As  usually  required  the  com- 
partments are  encircled  with  a  layer  of  cotton  netting  and  upon 
this  is  placed  the  canvas. 

The  Industrial  Filtration  Corporation,  which  manufactures 
the  Zenith  filter,  is  also  building  a  rotary  hopper  dewaterer, 
which  could  be  used  advantageously  for  dewatering  the  coarse 
coal  prior  to  drying  it  in  centrifugal  dryers.  The  Zenith  rotary 
hopper  dewaterer,  shown  in  Fig.  171,  acts  as  an  automatic  drain- 
ing board  and  dewaterer  all  in  one  and  fulfills  a  long  felt  want 
for  an  automatic  machine  for  dewatering  coarse  coal  in  a  more 
effective  way  than  can  be  accomplished  with  dewatering  elevators. 

The  rotary  hopper  dewaterer  consists  of  a  series  of  hoppers  or 


312 


COAL  WASHING 


deep  compartments  provided  with  filter  bottoms  and  arranged 
radially  about  a  central  shaft  and  hub  valve.  Each  hopper,  be- 
low the  filter  bed,  is  connected  to  a  valve  by  means  of  pipes  of 
ample  capacity.  Provision  is  made  in  the  valve  for  drawing  off 
the  mother  liquor  and  wash  water  separately  by  suction  (should 
it  be  desired  to  keep  the  two  separate)  and  for  pressure  (when 
needed)  for  discharging  the  contents  of  the  hoppers. 

The  design  of  the  valve  is  such  that  as  the  drum  rotates  suc- 


Fig.  171.     Rotary  Hopper  Dewaterer 

tion,  pressure  or  cut  off  is  applied  automatically  to  each  hopper 
as  it  reaches  the  proper  point  on  the  arc. 

The  rotary  hopper  dewaterer  thus  is  in  reality  a  series  of  sim- 
ple filtering  beds,  which  are  connected  so  that  they  act  continu- 
ously and  automatically. 

As  the  hoppers  rotate  they  are  charged  from  an  overhead  chute 
at  about  30  deg.  before  they  reach  the  top.  Suction  is  applied 
during  loading  and  in  the  course  of  filtration  each  hopper  passes 
through  an  arc  of  approximately  120  deg.  As  each  hopper 
passes  somewhat  below  the  horizontal,  suction  is  cut  off  and  a 


TREATMENT  OF  SLUDGE  313 

puff  of  air  or  steam  is  introduced  through  the  hub  valve :  the 
time  of  suction,  cut  off  and  pressure  being  automatically  con- 
trolled. 

This  machine  is  provided  with  the  same  patented  valve  as  is 
used  on  the  Zenith  rotary  filter  and  possesses  the  same  advan- 
tages arising  from  extra  large  pipe  lines.  Should  the  filtering 
medium  wear  out  in  one  compartment,  it  is  necessary  to  replace 
only  the  portion  covering  that  one  hopper.  Of  course  the  princi- 
pal consideration  that  commends  this  machine  is  the  fact  that  it 
is  automatic  and  continuous  in  operation  and  requires  no  labor. 
It  is  replacing  centrifugals,  gravity  and  pressure  machines  in 
many  places. 

The  Oliver  Continuous  Filter  consists  of  a  hollow  drum  sup- 
ported on  trunnions,  and  revolving  with  the  lower  part  sub- 
merged in  a  tank.  The  filtering  medium  is  on  the  outside  of  the 
drum  and  the  inside  is  water-tight.  The  space  between  the  two 
surfaces  is  divided  into  shallow  compartments,  each  virtually 
forming  an  independent  unit.  Arranged  radially  in  the  hollow 
interior  of  the  drum  is  a  system  of  pipes  connecting  each  com- 
partment with  the  automatic  valve,  which  controls  the  applica- 
tion of  vacuum  and  the  admission  of  compressed  air  or  steam. 

Material  to  be  filtered  is  maintained  at  a  certain  height  in  the 
filter  tank,  and  since  a  homogeneous  pulp  is  essential,  this  is  as- 
sured by  a  mechanical  agitator,  fitted  into  the  tank.  As  the 
drum  rotates,  the  filtering  surface  passes  through  every  part  of 
the  agitated  mass  and  a  cake  begins  to  build  up  on  each  compart- 
ment immediately  upon  entering  the  material,  this  process  con- 
tinuing until  it  emerges  from  the  pulp.  The  liquid  passes 
through  the  filter  medium  and  the  vacuum  pipes  to  the  automatic 
filter  valve,  which  controls  the  whole  cycle  of  operation.  This 
automatic  filter  valve  consists  of  a  flat  plate  with  a  number  of 
round  ports  corresponding  to  the  compartments  on  the  surface 
of  the  filter  drum.  The  pipes  for  vacuum  and  compressed  air 
connect  to  these  ports.  The  valve  chamber  has  annular  ports 
corresponding  to  the  different  stages  in  the  cycle  of  filter  oper- 
ation, such  as  forming,  washing,  drying  and  discharging  of  the 
cake.  The  face  is  ground  to  seat  accurately  with  the  valve  plate 
and  is  automatically  held  in  contact  with  it  by  the  existing 
vacuum  and  by  the  action  of  a  spiral  spring  when  there  is  tern- 


314 


COAL  WASHING 


porarily  no  vacuum.  An  adjusting  rod  prevents  the  automatic 
valve  turning,  and  insures  maintenance  of  the  desired  conditions 
and  adjustments. 

The  filter  cloth  is  not  applied  separately  to  each  compartment 
but  one  piece  covers  the  entire  drum,  wire  being  wound  spirally 
over  it  to  hold  the  cloth  firmly  in  place  and  protect  it  from  wear. 
With  this  arrangement  an  unbroken  cake  of  uniform  size  and 
consistency  is  constantly  being  formed.  The  vacuum  is  auto- 


Fig.  172.     Oliver  Filter 

matically  shut  off  and  just  before  reaching  the  scraper  com- 
pressed air  is  automatically  admitted  to  the  compartments  to 
loosen  the  cake.  The  scraper  completely  removes  the  cake  and  a 
perfectly  clear  filter  surface  enters  the  filter  tank,  thus  starting 
another  cycle.  The  discharged  cake  can  be  handled  upon  a  belt- 
conveyor,  or  dropped  directly  into  storage  bins. 

For  maintaining  a  vacuum  on  the  Oliver,  either  a  wet  or  dry 
vacuum  system  is  employed,  but  the  latter  is  found  the  more 
efficient  and  economical  in  most  cases.  In  addition  to  the  dry 
vacuum  pump,  a  vacuum  receiver  and  a  moisture  trap  are  em- 
ployed. A  centrifugal  pump  is  also  used  where  a  30  ft.  perpen- 
dicular drop  from  the  vacuum  receiver  is  not  available. 

Continuous  and  automatic  operation  is  the  keynote  of  Olive 


TREATMENT  OF  SLUDGE 


315 


efficiency.  Pulp  is  simply  fed  into  the  filter  tank  by  gravity, 
though  pumps  may  be  used  with  good  success,  and  nothing  fur- 
ther is  required  except  to  arrange  where  the  discharged  water 
and  coal  are  to  be  sent.  It  is  advisable  to  have  some  one  occa- 
sionally pass  near  the  equipment  to  see  that  it  is  operating  satis- 
factorily and  to  lubricate  it ;  but  it  is  not  necessary  to  have  an 
attendant  on  duty  at  all  times. 

The  discharged  coal  contains  15  to  18  per  cent,  of  moisture, 
and  the  capacity  is  from  500  to  1000  Ib.  dry  weight  of  coal  per 
sq.  ft.  of  filter  area  in  24  hr.  This  varies,  depending  on  the 
per  cent,  of  solids  in  the  filter  feed,  size  of  grains,  etc.  The 


Vacuum  Release. 


NOTE:- 

Moisture  Trap  is  recommended 
with  Pumps  of  all  sizes  but  may 
be  omitted  in  email  installations 


Oliver  Continuous  Filter  Dry  Vacuum  Pump 

Place  at  an?  Convenient 
Elevation 

Fig.    173.     Oliver   Filtering  and  Dewatering  System   in   Connection  with 

Dor  Thickener 


standard  type  of  Oliver  filter  for  general  use  is  shown  in  Fig.  172, 
while  Fig.  173  illustrates  a  filtering  Lnd  dewatering  system  in 
connection  with  a  Dorr  tank. 

The  American  Filter  shown  in  Figs.  174  and  175  is  of  a  some- 
what different  design.  It  has  a  series  of  filtering  disks  instead  of 
a  filtering  drum. 

In  the  American  filter  filtration  is  carried  on  by  applying  a 
vacuum  to  the  discharge,  thus  inducing  a  flow  of  filtrate.  The 
filter  elements — heavy  screen  filter  discs  of  sectionalized  con- 
struction— are  mounted  perpendicular  to  a  horizontal  shaft  pro- 
vided with  longitudinal  passageways  which  connect  to  all  leaf 
sectors  in  the  same  phase  of  rotation.  The  lower  half  of  the 
filter  leaves  dip  into  the  sludge  or  pulp  which  is  held  in  a  pan 


316 


COAL  WASHING 


construction  which  provides,  on  the  discharge  side  of  the  filter, 
individual  pans  for  each  leaf  so  that  solids  drop  down  between 


Fig.  174.     American  Filter 

pans  into  whatever  type  of  conveyor  is  provided.     The  discharge 
from  the  filter  is  controlled  by  a  rotating  valve  which  provides 


Fig.  175.     American  Filter  Installation 

for  separate  discharge  of  filtrate  and  effluent  wash-water.     A 
compressed  air  connection  is  also  provided  for  inflating  each  filter 


TREATMENT  OF  SLUDGE  317 

leaf  section  as  it  starts  to  pass  the  scraper.  When  the  solids  are 
to  be  washed  the  filter  is  provided  with  a  spray  washing  mecha- 
nism. The  operation  of  the  filter  is  continuous,  each  leaf  section 
in  turn  passing  through  a  period  of  filtering,  washing,  drying 
and  discharging. 

The  special  design  permits  installing  a  much  greater  filter  area 
in  the  same  floor  space  than  is  possible  with  any  other  type  of 
suction  filter.  Huge  diameters  are  unnecessary  and  the  filter  re- 
quires little  head  room.  The  filter  cloth  is  thoroughly  cleaned  at 


Fig.  176.     Lowden  Dryer 

each  revolution,  thus  maintaining  a  maximum  capacity  per  unit 
of  filter  area.  Ample  drainage  is  provided  from  the  filter  leaves, 
thus  insuring  a  dry  cake.  Any  single  leaf  section  may  be  easily 
removed  and  recovered,  thus  eliminating  long  shut-downs  for 
redressing. 

If  it  should  be  found  desirable  to  dry  the  sludge  to  less  than 
15  per  cent,  of  moisture  a  heat  dryer  could  be  installed  in  con- 
nection with  the  continuous  filters.  Such  a  machine,  which  is 
now  used  extensively  for  drying  the  flotation  concentrates  in  ore 
dressing  mills,  is  the  Lowden  patented  heat  dryer  illustrated  in 
Fig.  176. 

The  Lowden  patent  dryer  was  designed  especially  to  meet  the 
urgent  necessity  for  a. machine  capable  of  handling  extremely 


318  COAL  WASHING 

fine  material  such  as  would  suffer  excessive  dust  losses  in  dryers 
of  other  types,  also  material  in  a  plastic  condition  such  as  would 
be  almost  impossible  to  feed  to  most  other  dryers. 

The  relatively  low  efficiency  of  the  pre-existing  hearth-type 
dryers  has  been  largely  overcome  in  this  machine,  and  it  offers 
advantages  over  any  other  on  many  classes  of  work. 

It  consists  of  a  hearth  composed  of  heavy  cast  iron  plates 
heated  from  beneath  by  the  gases  of  combustion  from  a  fire  box 
or  by  waste  heat.  The  material  is  normally  delivered  to  the 
cooler  end  of  the  hearth  and  is  slowly  advanced  to  the  hotter 
discharge  end  by  a  series  of  rabbles  which  effectively  break  up 
and  plow  the  material,  thus  exposing  it  thoroughly.  It  is  the 
peculiar  and  characteristic  action  of  its  rabbling  mechanism 
which  sharply  distinguishes  this  device  from  all  other  analogous 
dryers  and  makes  it  successful  where  others  fail.  The  rate  of 
transmission  of  heat  through  cast  iron  is  as  rapid  as  can  be  ab- 
sorbed by  the  material  in  the  vaporization  of  moisture,  if  the  cast 
iron  plates  can  be  kept  free  from  a  layer  of  insulating  material. 
This  the  raking  mechanism  of  the  Lowden  dryer  accomplishes, 
and  the  objections  to  the  grasshopper  dryer  in  that  respect  are 
absent. 

The  possibility  of  delivering  large  batches  of  insufficiently 
dried  material,  which  is  always  present  with  the  chain  rabbled 
dryers,  is  also  absent,  owing  to  the  reciprocating  action  of  the 
rakes.  Any  material  that  adheres  to  the  rabbles  falls  back  upon 
the  hearth  in  practically  the  same  place  as  that  from  which  it 
was  lifted. 

The  Lowden  patent  dryer  is  built  in  various  sizes  for  capaci- 
ties of  10  tons  up  to  100  tons  per  24  hr.,  the  size  for  any  given 
capacity  depending  upon  the  amount  of  moisture  to  be  elimi- 
nated, and  in  lesser  degree  upon  other  factors.  The  speed  is 
low,  two  cycles  or  less  per  minute,  and  the  power  required  is 
small. 

In  Fig.  177  one  of  the  many  possible  arrangements  of  the  Low- 
den dryer  in  connection  with  a  continuous  filter  in  a  unit  for 
drying  sludge  is  illustrated. 

The  thickened  sludge  is  drawn  from  the  Dorr  thickener 
through  "D"  by  a  diaphragm  pump  and  flows  at  "T"  into  the 
sludge  tank  of  a  continuous  filter,  from  which,  after  dewatering, 


TREATMENT  OF  SLUDGE 


319 


it  falls  at  "F"  upon  the  hearth  of  the  Lowden  dryer,  being  dis- 
charged at  "G." 

For  the  sake  of  clearness,  the  illustration  below  shows  the 
equipment  arranged  in  a  line,  but  ordinarily  it  would  be  so  dis- 
posed that  the  coarse  concentrates  bin  would  be  near  the  dis- 
charge of  the  dryer. 


* 


177. 


Installation    of    Lowden    Dryer    in    Connection    with    Portland 
Filter  and  Dor 'Thickener 


The  installation  of  the  Lowden  dryer  offers  no  special  difficul- 
ties, either  in  feeding  or  disposition  of  the  dried  product.  It 
may  be  fed  direct  from  a  filter,  as  shown,  or  from  a  conveyor, 
and  may  discharge  upon  a  conveyor,  directly  into  a  bin  or  car, 
or  into  the  boot  of  an  elevator. 


CHAPTER  XXXI 
PYRITE  RECOVERY 

Pyrites  are  found  in  coal  either  in  the  form  of  sulphur  balls 
or  in  the  shape  of  fine  scales  and  grains  disseminated  throughout 
the  mass.  The  separation  of  the  pyrites  from  the  coal  does  not 
offer  any  appreciable  difficulties  on  account  of  the  great  differ- 
ence in  the  specific  gravities  of  the  two  materials.  The  specific 
gravity  of  pyrites  is  from  4.9  to  5.2,  and  even  the  slate  carrying 
fine  flakes  of  sulphur  has  a  specific  gravity  of  only  slightly 
below  3. 

A  more  serious  problem  is  how  to  prepare  the  pyrite  if  it  occurs 
in  considerable  quantities.  This  can  be  best  accomplished  by  wet 
separation,  and  the  following  methods  are  used: 

1.  If  the  pyrite  appears  in  large  pieces  or  is  contained  within 
large  pieces  of  slate,  hand  picking  and  subsequent  separation  into 
pure  pyrite  and  mixed  products  is  advisable. 

2.  Instead  of  hand  picking,  the  heavy  pyrite  can  also  be  re- 
covered in  coarse  coal  jigs,  which  have  an  auxiliary  screen  slop- 
ing toward  the  center.     The  pyrite  is  removed  from  the  lowest 
point  of  the  screen  through  a  kettle  valve.     In  some  instances  nut 
coal  jigs  have  a  separate  bed  for  the  separation  of  the  pyrite  and 
three  products  are  made  in  the  following  manner:     (a)  Pyrites 
through  an  artificial  bed  and  screen  into  the  hutch;    (b)   slate 
through  a  slate  gate,  located  at  a  somewhat  higher  level,  and  (c) 
clean  coal  overflowing  in  front  of  the  jig. 

3.  Rewashing  of  the  refuse  is  a  method  especially  advisable 
for  large  size  pyrites. 

4.  For  fine  pyrite  the  methods  under   (b)    and   (c)    can  be 
adapted  by  using  a  fine  coal  jig. 

5.  If  the  pyrite  is  so  finely  disseminated  that  it  partially  goes 
over  with  the  sludge,  it  settles  out  in  the  clearing  basins  and  the 
sludge  rich  in  pyrite  can  be  treated  on  tables.     On  account  of 
the  small  quantities  of  pyrite  in  coal  the  economic  results  gained 

320 


PYRITE  RECOVERY  321 

by  its  recovery  usually  lie  within  narrow  limits.  The  great  price 
fluctuations  of  sulphur  are  also  discouraging,  and  under  normal 
conditions  a  lasting  profitable  operation  is  at  best  doubtful. 

C.  H.  Cady,  geologist  of  the  Illinois  Geological  Survey,  has 
studied  the  occurrence  of  pyrite  thoroughly  and  has  classified  the 
pyrite  found  in  Illinois  coal  as  follows : x 

"Pyrite  has  been  observed  to  have  the  following  habits  of  occurrence: 
As  brassy,  massive,  metallic-appearing  mineral  without  apparent  crystalline 
structure  or  form;  as  a  crystalline  mineral;  as  a  brown  or  gray  mineral 
without  metallic  luster  or  apparent  crystalline  character,  this  form  being 
commonly  laminated;  and  as  impregnations  in  a  very  fine  state  and  prob- 
ably crystalline.  The  material  occurs  in  the  following  common  forms: 
As  balls  and  lenses  of  a  well  defined  shape  and  easily  separable  from  the 
surrounding  coal;  as  balls  and  lenses  with  the  outer  parts  more  or  less 
ramifying  into  the  surrounding  coal  and  hence  not  easily  separated  from  it; 
as  a  fine  leaf  mineral  in  finely  divided  state  lying  along  innumerable  joint 
cracks  in  isolated  patches  of  the  coal ;  as  typical  vein  filling,  especially  in 
'horsebacks';  as  replacement  of  limestone,  forming  'niggerheads"  in  the 
roof  shale,  and  in  other  limestone  masses  found  associated  with  the  coal; 
as  impregnations  of  mother  coal  and  of  the  clay  filling  of  horsebacks;  as 
balls  in  the  floor  clay;  as  plates  or  sheets  commonly  found  in  the  partings 
between  benches;  as  facings  in  joint  cracks,  commonly  very  thin  plates; 
and  as  rosettes  in  the  laminations  of  the  black  fissile  shales  found  above 
some  of  the  coals. 

"The  habit  of  occurrence  of  the  pyrite  seems  to  bear  relation  to  the  form. 
Pyrite  in  balls  and  lenses  easily  separated  from  the  coal  is  apparently 
nearly  always  of  the  brassy,  massive  variety.  The  lenses  and  balls  of  in- 
definite boundary  are  commonly  the  gray,  stony  variety;  this  variety,  at 
least,  seems  always  to  have  an  indefinite  outline.  The  plate  and  sheet 
pyrite  is  variable  in  its  habit,  but  pyrite  of  metallic  appearance  seems  to 
be  the  most  common  variety.  Facings  are  composed  of  the  bright  pyrite. 
Vein  fillings,  the  nodules  in  the  fireclay,  the  rosettes  in  the  roof  slate  and 
probably  the  impregnations  of  the  clay  fillings  of  horsebacks  and  of  mother 
coal  are  all  of  a  crystalline  nature.  Pyrite  which  replaces  limestone  takes 
on  the  form  and  texture  of  the  original  rock. 

"The  ease  with  which  pyrite  is  separated  from  coal  at  the  face,  the  tipple 
or  the  washery  depends  largely  upon  the  form  of  occurrence.  As  between 
the  stony,  crystalline  and  massive  bright  varieties  there  is  practically  no 
distinction  so  far  as  relative  ease  of  recovery  is  concerned.  The  most  easily 
separable  pyrite  is  that  occurring  in  balls  and  lenses  of  the  brassy  variety. 
It  is  plainly  seen  and  its  outline  clearly  defined,  so  that  it  is  usually 
broken  out  by  the  miner  at  the  face.  The  pyrite  occurring  in  the  nigger- 

i  "Valuable  Pyrite  in  Illinois  Coal  Beds,"  by  G.  H.  Cady,  geologist  in 
charge  of  coal  studies,  State  Geological  Survey  Division,  Urbana,  111.  (Coal 
Age,  Vol.  10,  No.  4,  1919.) 


322  COAL  WASHING 

heads  and  in  limestone  lenses  or  masses  in  the  coal  or  near  the  boundary  of 
the  coal  and  the  roof  rock  are  also  readily  discarded.  Next  in  relative 
ease  of  removal  is  the  plate  or  sheet  pyrite,  provided  the  plates  are  of 
sufficient  thickness  to  withstand  the  shattering  effect  of  mining.  If  Vz  in. 
or  more  thick,  the  plates  can  usually  be  removed  without  difficulty  from 
the  coal,  in  pieces  sometimes  more  than  a  foot  wide.  As  the  seam  com- 
monly parts  at  the  pyrite  band  the  material  can  be  removed  rather  easily. 
Small  pieces,  however,  commonly  remain  in  the  coal.  If  the  plates  or 
sheets  are  thin,  the  proportion  that  is  recoverable  is  small,  since  it  is  com- 
monly so  badly  shattered  in  mining  that  removal  by  the  miner  is  practically 
impossible.  Such  pyrite  as  this  could  be  largely  removed  by  washing  the 
finer  sizes  of  coal. 

"The  removal  of  the  brown,  or  gray,  banded  pyrite  in  the  mine  is  at- 
tended by  more  or  less  difficulty.  It  is  not  quite  as  readily  seen  as  the 
bright  variety,  for  not  uncommonly  it  is  rather  dark  colored  by  reason  of 
the  presence  of  a  large  quantity  of  what  appears  to  be  carbonaceous  matter. 
Then,  also  its  outlines  are  indefinite.  To  remove  this  variety  of  pyrite 
much  coal  must,  in  general,  be  wasted  if  the  entire  mass  of  the  lens  is  to 
be  recovered.  Coals  having  this  form  of  pyrite  in  large  quantities  are 
almost  sure  to  have  a  rather  high  pyrite  content  as  shipped,  unless  all  the 
coal  is  washed.  If  the  larger  sizes  of  coal  were  hand-picked  at  the  tipple, 
large  amounts  of  this  material  would  probably  be  effectively  removed. 
Pyrite  present  as  facings  is  practically  impossible  of  removal  by  any  method 
of  hand-picking  except  where,  as  in  some  rare  localities,  the  facings  become 
so  numerous  as  to  be  practically  a  mass. 

"In  some  of  the  better  Illinois  coals  pyrite  occurs  only  as  facings  or  as 
leaf  pyrite.  The  removal  of  some  of  this  impurity  can  be  accomplished  by 
crushing  and  washing  the  finer  sizes,  but  it  is  probable  that  the  actual 
amount  of  pyrite  that  could  be  thus  removed  would  be  negligible  and  would 
only  in  small  degree  affect  the  selling  value  of  the  coal. 

"Masses  of  leaf  pyrite  are  commonly  not  discarded;  although  the  mass 
may  have  a  bright  appearance,  the  actual  amount  of  pyrite  present  is  small. 
This  is  indicated  by  the  fact  that  such  a  mass  of  coal  filled  with  particles 
of  leaf  pyrite  weighs  but  little  more  than  pure  coal.  Furthermore,  such 
pyrite  is  difficult  to  separate  by  washing,  the  small  flakes  of  mineral  re- 
maining suspended  and  floating  off  with  the  coal.  The  problem  of  sepa- 
rating such  pyrite  from  coal  is  yet  to  be  solved. 

"The  vein  pyrite  coal  in  Illinois  rarely  exceeds  J/2  in.  in  thickness.  Its 
occurrence  is  practically  restricted  to  the  horseback  fissures  such  as  are 
found  to  be  especially  numerous  in  the  No.  5  bed.  The  coal  adjacent  to 
such  fissures  is  commonly  well  impregnated  with  pyrite  in  a  finely  divided 
state  so  that  the  entire  mass  is  very  hard.  It  is  the  common  practice  to 
entirely  discard  the  mass  of  coal  attached  to  the  sulphur  'spar,'  as  it  is 
called,  for  it  is  usually  thoroughly  impregnated  with  pyrite.  The  miner 
receives  extra  pay  for  the  removal  of  this  material  so  that  impurity  of  this 
sort  does  not  commonly  reach  the  top,  except  where  the  'spars'  are  thin. 

"Clay  veins  also  are  commonly  rich  in  a  finely  divided  pyrite  that  is  dis- 


PYRITE  RECOVERY  323 

seminated  throughout  their  mass  and  reaches  out  into  the  adjacent  coal. 
This  pyrite  with  the  attached  coal  is  discarded  just  as  the  pyrite  and  coal 
in  sulphur  'spars'  is  thrown  away.  In  many  mines  the  removal  of  the 
horsebacks  is  a  cause  of  considerable  waste,  and  in  some  instances  serious 
consideration  could  well  be  given  to  the  problem  of  its  elimination  in,  at 
least,  a  large  degree. 

"The  impregnation  of  mother  coal  by  pyrite  gives  a  very  hard  black  ma- 
terial with  the  general  appearance  of  mother  coal  but  with  a  slight  golden 
tinge.  The  material  is  very  hard.  The  substance  is  commonly  called 
'blackjack'  by  the  miners,  though  it  is  possible  that  all  the  'blackjack'  of 
miners  is  not  pyritized  mother  coal.  The  material  is  nearly  as  difficult  to 
cut  as  the  gray  or  brassy  pyrite,  and  where  it  lies  in  relatively  large 
masses  is  readily  discarded.  Smaller  masses,  however,  especially  if  im- 
bedded in  large  masses  of  coal,  are  less  easily  removed.  'Blackjack'  com- 
monly sticks  rather  tightly  to  the  surrounding  coal  and  the  removal  of 
pieces  less  than  a  foot  in  length  and  an  inch  or  two  thick,  except  as  they 
occur  along  partings,  does  not  seem  to  be  common  practice. 

"The  sulphur  balls  found  in  the  floor  clay  and  the  pyrite  rosettes  found 
in  the  roof  shale  do  not  commonly  get  into  the  coal  as  shipped.  They  are 
rather  interesting  occurrences  but  of  no  special  importance  commercially, 
except  that  clays  with  these  sulphur  concretions  are  not  adapted  for 
burning." 

Professor  E.  A.  Holbrook  carried  on  a  series  of  tests  for  the 
purpose  of  determining  the  best  method  of  recovering  the  pyrites 
from  coal  by  crushing  and  washing.1 

"Since  the  hand-picked  pieces  of  pyrite  from  coal  range  up  to  several 
inches  thick  and  more  than  a  foot  square,  and  since  pieces  of  coal  tend  to 
adhere  to  the  lumps,  hand-picking  in  general  will  not  produce  a  high  grade 
product.  It  is  true  that  by  hand-picking  and  hammering  the  larger  lumps 
may  be  freed  of  coal  sufficiently  to  produce  a  salable  product,  but  this 
method  involves  the  waste  of  the  large  amount  of  pyrite  which  occurs  in 
pieces  smaller  than  2  in.  in  diameter,  or  of  a  size  too  small  to  permit  hand- 
picking  to  be  done  profitably.  It  should  be  remembered  also  that  the  fine 
pyrite  is  of  greater  value  per  ton  than  the  coarse  material. 

"Since  the  specific  gravity  of  the  pyrite  is  high  (4.7  to  5.1)  as  compared 
with  that  of  coal  (1.3),  washing  by  a  process  involving  jigging  or  agita- 
tion in  water  causes  the  heavy  mineral  to  sink  rapidly  while  the  light  ma- 
terial may  be  drawn  off  at  the  top.  This  principle  of  separation  is  used  in 
the  ordinary  jig. 

"With  the  purpose  of  devising  some  simple  washing  or  ore-dressing 
process  to  effect  a  separation  of  the  pyrite  from  its  adhering  coal,  the  De- 
partment of  Mining  Engineering  of  the  University  of  Illinois  has  under- 

i  "The  Utilization  of  Pyrite  occurring  in  Illinois  Bituminous  Coal,"  by 
E.  A.  Holbrook.  Circular  Xo.  5,  Engineering  Experiment  Station,  Univer- 
sity of  Illinois,  Urbana,  Illinois. 


324  COAL  WASHING 

taken  a  series  of  tests,  with  various  samples  of  pyrite.  As  a  result  of  these 
experiments,  an  arrangement  of  machines  has  been  worked  out,  and  the 
power  required  and  the  cost  of  operation  have  been  determined  for  a  simple 
plant  capable  of  preparing  nearly  pure  pyrite  on  the  one  hand  and  com- 
mercial coal  on  the  other. 

"The  mining  laboratory  at  the  University  of  Illinois  is  equipped  with 
rock  crushers,  breakers,  and  rolls  of  several  different  kinds,  installed  in 
such  a  manner  as  to  make  possible  the  determination  of  the  best  method  of 
crushing  any  ore  or  coal  to  the  size  necessary  for  subsequent  treatment. 
Included  in  this  equipment  are  screens  of  the  revolving  or  trommel  type, 
and  shaking  and  vibrating  screens  to  divide  the  crushed  material  into  sev- 
eral sizes  required  for  further  treatment.  There  are  also  jigs  of  the 
plunger,  Hartz  or  Liihrig  type  together  with  jigs  of  the  basket  or  Stewart 
type.  These  jigs  separate  the  valuable  mineral  from  the  refuse.  In  addi- 
tion special  machines  in  the  form  of  concentrating  tables  are  installed  for 
special  treatment  of  fine  or  small  material,  that  is,  material  too  small  to  be 
successfully  handled  by  jigs. 

"Preliminary  tests  were  made  by  crushing  the  crude  pyrite  to  various 
sizes  in  different  types  of  crushing  machinery  such  as  breakers,  rolls,  and 
pulverizers,  and  by  comparing  the  various  samples  of  pyrite  to  determine 
the  extent  to  which  separation  of  pyrite  and  coal  had  been  effected.  Sizing 
tests  were  made  on  various  kinds  of  screens  and  separation  of  these  prod- 
ucts was  effected  by  different  types  of  jigs,  washers,  and  concentrating 
tables.  The  Delamater  float  and  sink  test  machine  was  particularly  useful 
in  determining  roughly  the  possibilities  of  separation  of  various  sizes  of 
mixed  coal  and  pyrite.  The  possibility  of  separating  pyrite  from  coal  by  a 
strong  electric  magnet  was  also  tried,  but  under  the  influence  of  a  6-ampere 
40-ohm  electric  magnet  installed  in  a  Dings  electromagnetic  separator,  the 
results  were  negative.  Without  describing  in  detail  the  various  tests,  it  is 
sufficient  to  give  an  outline  and  to  present  the  average  results  of  those  tests 
which  proved  most  successful,  and  which  gave  a  high  percentage  of  recovery 
at  a  low  or  reasonable  cost. 

SUMMARY  OF  TESTS 

"Machinery  Required.  The  tests  performed  lead  to  the  conclusion  that 
the  practical  separation  of  pyrite  from  Illinois  coal  for  the  purpose  of  ob- 
taining a  commercial  grade  of  pyrite,  with  coal  as  a  by-product,  presents 
no  difficulty  when  performed  by  crushers,  screens,  and  concentrating  ma- 
chines adapted  to  ordinary  ore-dressing  work.  The  chief  problem  is  to 
secure  a  plant  of  the  greatest  simplicity  and  of  the  lowest  cost.  At  the 
same  time  it  should  be  of  good  capacity  and  should  yield  a  high  percentage 
of  recovery  of  the  pyrite. 

"Percentage  of  Recovery.  The  experiments  from  which  the  data  resulted 
indicate  that  a  simple  plant  will  recover  about  81  per  cent,  of  the  pyrite 
in  the  coal,  and  that  if  the  middlings  product  from  the  jig  is  crushed  and 
retreated,  this  recovery  can  be  increased  to  about  87  per  cent.  This  pyrite 


PYRITE  RECOVERY  325 

will  average  more  than  40  per  cent,  of  sulphur  and  may  be  sold  directly  to 
chemical  or  to  fertilizer  companies.  The  coal  recovered  as  a  by-product  is 
not  greatly  inferior  to  ordinary  screenings." 

ESTIMATED  OPERATING  STATEMENT  OF  A  PYRITE  PLANT  OF  A  CAPACITY  OF 
50  TONS  PER  8  HOUR  DAY 

Debit  Credit 

50  tons  of  hand-picked  pyrite  Coarse  pyrite,  24.000  lb.,  45 

at  $1.35    $77.50  per    cent,    sulphur    at    15 

Interest  and  depreciation  on  cents   per  unit:    $6.75   per 

plant.     Investment  of  $18,-  ton    $  81.00 

000    at    20    per    cent,    per  Pea  pyrite,  22,000  lb.,  45  per 

year    12.00          cent,   sulphur   at    15   cents 

La'bor,  5  men  at  $3 15.00          per  unit   74.25 

Supplies  and  renewals 15.00      Fines,  6,510  lb.,  41  per  cent. 

Power,  50  h.p 10.00          sulphur    at    15    cents    per 

unit 20.02 

$129.50      Extra  pyrite  if  middlings  are 
retreated:    4,290    lb.,    43.5 

per  cent,  sulphur 13.99 

Coal,  35,231  lb.,  at  $1  per  ton     17.62 
Loss    (allowing    for    coal    in 
middlings    as    loss),    8,325 
lb. 

$206.88 
129.50 

Profit  per  day $  77.38 

Profit  per  ton  of  raw  pyrite. $     1.55 

TABLE  51 

Estimated  Operating  Results.  An  effort  has  been  made  to 
forecast  the  results  of  operating  a  50  ton  per  8  hour  day  pyrite 
plant,  under  conditions  comparable  to  the  average  met  at  Illinois 
mines,  where  pyrite  is  to  be  found  in  sufficient  quantities  to  war- 
rant recovery.  The  summary  presented  in  Table  51  is  based 
partly  upon  estimated  figures,  especially  with  reference  to  the 
cost  of  crude  pyrite  as  laid  down  at  the  plant.  The  figures  given 
for  the  value  of  the  product  are  based  on  a  price  of  15  cents  per 
unit  of  sulphur.  In  nearly  every  case,  the  estimates  are  believed 
to  represent  maximum  costs  and  conservative  selling  prices. 

Method  of  Operation.  Fig.  178  is  a  diagrammatic  illustration 
or  simplified  flow  sheet  of  the  treatment  plant  recommended  as  a 
result  of  the  tests  performed.  The  successive  steps  believed 
essential  to  the  complete  treatment  of  such  pyrite  are  shown. 
Fig.  179  shows  the  same  flow  sheet  with  percentages  of  recovery 


326 


COAL  WASHING 


attained  at  each  part  of  the  process.  This  indicates  results 
which  might  be  accomplished  in  practical  work.  Owing  to  the 
difficulty  experienced  in  regulating  machines  for  the  relatively 
small  tonnage  treated  in  laboratory  work,  it  is  believed  that  in 
every  case,  commercial  practice  on  a  large  scale  would  result  in 

Hand  Picked  Pyrite 


T 

Water 

Disint 

t          *          .. 
egratiug  Screen  (iM  )                     y 

' 

r 

Underize 

^1 
Oversize           | 

evator 
k           X"  Trommel  Screen 

Clean  Coarse 
Pyrite 

Over(  Ji  to  1H) 

Beady  for  Shipment 
Under  (Ji") 

Barz  Jig 

Wilfley  Table 

Tailings                       Concen 

~v    ~\ 

Tailings        Concentrates 

trates 

• 

Fine 
4te_ 

Middlings 

(Beady      Clean 

~r 

^                      Rolls  to  H 

Clean  Coal   (Ready) 
Screeuiugs  Size    (for  Use) 

~]   f 

\ 

Clean  Coal   (Ready)        { 
No.5  Nut       ('or  Use)     j 

t           Clean  Medium 
PyriU 

T                         (  Beady  for  Shipment  ) 
Water  Settlement 

F                   > 

Sludge  (To  Waste)                   Clean  Water 

Fig.   178.     Flow  Sheet  for  Pyrite  Recovery  Plant 

higher  recovery  than  is  indicated  by  this  outline.  The  tonnage 
is  based  on  a  plant  capable  of  treating  50  tons  of  crude  hand- 
picked  pyrite  per  8  hour  day,  as  this  is  believed  to  represent  the 
largest  plant  needed  by  one  mine  or  even  by  several  mines  com- 
bined. 


PYRITE  RECO VER Y 


327 


RAW  PYRITE 
60  Tons  per  8  Hour  Day 

Crushed  and  Screened 


Less  than  Ji  inch  diam. 

Fines 

10  5  Tons  21* 
21.000  Ib. 


More  than  1H  inches  diam. 

Coarse  Pyrlte 

12  Tons  24  * 

45.4*  Sulphur 

108%  Ib.  Sulphur 


Pyrite  (Concentrates)  Coal 
31  %  63* 

6610  Ib.  13231  Ib. 

41-05*  Sulphur 
2672  Ib.  Sulphur 


Losses 

6* 

1260  Ibs. 

8.1*  Sulphur  8.1*  Sulphur 
1061  Ib.  102  Ib.Sulphur 
Sulphur 
17*  Ash 


Medium  Sizes 

Ji'to  1^'inch 

27.5  Tons  55* 

55000  Ib. 


Pyrite  (  Concentrates)  Middlings        Tailings  (Coal)  Louaes 

40*  15*  40*  5* 

22000  Ib       8250  Ib.       22.000  Ib        2750  Ib. 

45.1*  Sulphur  27.4*  Sulphur      8  3*  Sulphur        83*  Sulphur 

0922  Ib.  Sulphur  22CO  Ib.  Sulphur  1826  Ib.  Sulphur  288  Ib.  Sulphur 

17*  Ash  17*  Ash 


Fig.  179.     Flow  Sheet  for  Pyrite  Recovery  Plant  Showing  Percentages  of 

Recovery 


JIG  MIDDLINGS    (  Mixed  Coal  and  Pyrite  ) 

8250  Ibs. 
27.4*  Sulphur 
2200  Ibs.  Sulphur 


Crush  to  y.''ln  Rolls 


> 

1 

1 

Pyrite 

Coal  and 

Loss 

52* 

\ 

4290  Ibs. 

£ 

43.5*  Sulphur 
1866  Ibs.  Sulphur 

1 

~T 

\ 

Coal.3580  Ibs. 

Coal  loss 

Loss 

8.2*  Sulphur 

311  Ibs. 

Sulphur  not 

325  Ibs.  Sulphur 

8*  of  the 

accounted  for 

17*  Ash 

Coal 

69  Ibs. 

Fig.  180.     Flow  Sheet  for  Recrushing  the  Jig  Middlings 


328 


COAL  WASHING 


Fig.  180  is  a  diagram  which  indicates  the  possibilities  of  re- 
crushing  the  middlings  product  obtained  from  the  second  com- 
partment of  the  jig,  then  screening  it  through  the  a/4  in.  trommel 
screen,  and  allowing  it  to  pass  either  to  the  jig  or  to  the  concen- 
trating table,  according  to  its  size.  In  this  way  the  recovery 
can  be  increased  by  about  6.4  per  cent. 

Table  52  shows  the  amount  of  percentage  of  recovery  or  loss 
of  the  pyrite  in  each  operation  of  the  process,  based  on  sulphur 
content  as  determined  by  sampling  and  by  analysis  of  each  of  the 
products  recovered. 

FLOW  SHEET  REDUCED  TO  A  BASIS  OF  SULPHUR  CONTENT,  SHOWING  THE 

AMOUNT  OF  SULPHUR  IN  EACH   PRODUCT   BASED   ON   THE   OUTPUT 

OF  A  PLANT  HAVING  A  CAPACITY  OF  50  TONS  PER  8-HoiiR  DAY 


Product 

Name 

Size 

Sulphur 
Per  Cent. 

Sulphur 
Content 
Lb.* 

Recovered 
Lb. 

phur    Recov- 
ered or   Lost 

Coarse    (Screen) 
Concentrates 
Fine  (Table) 
Concentrates 
Medium  (Jig) 
Concentrates 

Total 

Lump 
Pyrite 
Fine 
Pyrite 
Pea 
Pyrite 

Above    I1/!  in. 
ring 

Under    %    in. 
IV?  in.  to 
%  in. 

45.4 
41.05 
45.1 

10896 
2672 
9922 

10896 
2672 
9922 
23490 

37.6 

9.2 

34.2 
81.0 
Recovery 

Middlings     from 

Medium  Crushed  to 

Concentrates  %  in. 

Total  with 
Middlings  Added 

Fine  Coal        No.    5    Nut  Under   %   in. 
Coal  from  Jig       Screening      lV->  in.  to 
Overflow  Size  ^  in. 

Loss  (Jig)  Coal  Estimated 

Loss  (Table)  Coal  Estimated 

Loss  (Middlings)      Pyrite      Not  acct.    for 
Total  Sulphur  in, 
Original  Product 


8.1 

8.3 

8.3 

8.1 

69  Ib. 


2260 


1826 

1061 
102 
238 


28977 


1866 
25356 


87.4 
Recovery 


*  28,977  Ib.  of  sulphur  from  50  tons  of  material  amounts  to  28.98  per  cent,  of  sul- 
phur in  original  crude  hand-picked  pyrite  (assuming  all  sulphur  to  be  in  the  form  of 
pyrite).  For  pyrite  containing  53.4  per  cent,  of  sulphur,  the  total  pyrite  content  of 
the  crude  pyrite  would  be  54,264  Ib.,  or  54.26  per  cent,  of  pyrite,  and,  therefore,  the 
content  of  coal  and  contained  ash  and  shale  ,/ould  be  45.74  per  cent.  The  total  re- 
covery from  disintegrating  screen,  jig,  and  table  on  the  crushed  crude  pyrite  was 
81.0  per  cent,  of  the  total  pyrite  or  23,490  Ib.  of  sulphur  from  the  28.977  Ib.  con- 
tained in  100,000  Ib.  of  crude  pyrite.  If  the.  middling  product  from  the  jig  is  re- 
crushed  and  treated,  the  recovery  is  increased  to  87.4  per  cent.,  or  25,356  Ib. 

TABLE  52 


The  coal  produced  as  a  by-product  contains  about  8  per  cent, 
of  sulphur,  a  part  of  which  is  in  the  form  of  pyrite.  In  commer- 
cial operations  extending  over  considerable  periods  this  loss  of 
pyrite  could  be  decreased,  as  it  is  largest  when  starting  and 
while  shutting  down  tLe  machinery.  These  operations  occur  f re- 


PYRITE  RECOVERY  329 

quently  in  experimental  runs.  The  amount  of  coal  recovered  as 
a  by-product  is  considerable,  the  tests  indicating  38,811  Ib.  per 
day  from  the  plant  and  product  under  discussion,  or  from  18 
to  20  tons.  It  should  be  remembered  that  this  coal  is  of  screen- 
ing size,  and  that  its  purity  depends  largely  upon  the  care  with 
which  the  pyrite  is  removed  during  the  process  of  cleaning. 

The  Tests  and  the  Results.  In  the  final  tests  the  pyrite  as 
received  (about  a  ton  in  weight)  contained  from  25  to  28  per 
cent,  of  sulphur,  or  about  50  per  cent,  by  weight  of  pyrite.  The 
other  50  per  cent,  of  the  mineral  consisted  of  coal  adhering  to 
the  lumps  and  intermixed  with  the  bands  of  pyrite.  The  mate- 
rial had  been  hand-picked  at  a  tipple  preparing  No.  7  coal  in 
the  Danville  district.  The  lumps,  including  the  adhering  coal, 
were  as  large  as  6  or  8  in.  in  thickness  and  were  often  a  square 
foot  in  area.  This  material  was  first  put  through  an  ordinary 
rock  breaker.  The  rock  breaker  in  the  laboratory  is  of  the  Gates 
gyratory  type,  but  from  tests  made  with,  a  Blake  type  rock 
breaker  it  is  believed  that  the  latter  type  will  be  equally  satis- 
factory and  probably  cheaper  in  first  cost.  Attention  is  here 
called  to  the  fact  that  ordinary  coal-crushing  machinery  is  not 
suitable  for  crushing  raw  pyrite.  The  pyrite  is  extremely  hard 
and  only  breakers  designed  for  hard  rock  should  be  used. 
Breakers  designed  for  soft  material  do  not  possess  adequate 
strength,  and  the  wear  will  be  excessive  if  used  on  this  class  of 
material. 

The  breaker  was  set  with  a  throat  opening,  or  discharge,  about 
1%  in.  wide,  and  although  the  pieces  discharged  through  this 
had  a  thickness  of  not  more  than  l1/^  in.,  the  area  of  some  of  the 
lumps  was  several  square  inches  in  extent,  owing  to  the  tendency 
of  the  pyrite  to  break  into  flat  slabs.  Examination  showed  that 
this  breaking  process  caused  a  large  portion  of  the  lump  pyrite 
to  separate  from  the  adhering  coal.  The  coal  itself  tended  to 
break  into  cubical  pieces.  Also  the  coal,  because  of  its  brittle- 
ness,  generally  broke  up  into  finer  sizes  than  the  pyrite. 

After  breaking,  the  large  lumps  of  pyrite  had  only  small  bits 
of  coal  adhering  to  them.  Thus  it  was  decided  to  screen  this 
material  in  an  attempt  to  secure  a  coarse  pyrite  which  would  be 
sufficiently  clean  for  the  market.  The  crushed  material  was  put 
through  a  revolving  or  trommel  screen  having  round-hole  open- 


330  COAL  WASHING 

ings  of  about  the  same  diameter  as  the  opening  in  the  rock 
breaker.  Since  the  coal  tended  to  break  into  cubical  pieces 
while  the  pyrite  tended  to  break  into  flat  pieces,  it  was  thought 
that  a  separation  could  be  made  of  the  two  by  simple  screening 
alone.  This  expectation  was  borne  out  by  results  obtained. 
Later,  steel  lifters  were  introduced  in  the  revolving  screen. 
During  screening  these  lifters  caused  the  material  to  be  carried 
to  the  top  of  the  screen  and  to  be  dropped  several  feet.  The 
impact  from  this  fall  served  to  break  any  large  coal  so  that  it 
passed  through  the  screen,  and  it  also  freed  the  pyrite  of  any 
small  particles  of  adhering  coal.  It  was  shown  also  that  lump 
pyrite  may,  if  desired,  be  further  cleaned  by  screening  the  ma- 
terial while  wet,  that  is,  by  introducing  sprays  of  water  into  the 
screen.  The  rubbing  action  of  the  wet  material  against  the 
screen  serves  to  loosen  most  of  the  specks  of  coal  remaining  on 
the  coarse  pyrite  so  that  they  may  pass  through  the  screen.  The 
greater  the  diameter  of  the  screen,  that  is,  the  greater  the  length 
of  fall  of  the  particles  after  having  been  lifted,  the  freer  is  the 
oversize,  or  clean  coarse  pyrite  of  coal  impurity. 

The  Size  of  Screen  Holes.  As  previously  mentioned  the 
largest  size  of  screen  opening  was  about  a  l1/^  in.  round  hole. 
This  screen  is  not  unlike  the  Bradford  disintegrator  which  is  in 
-common  use  for  cleaning  coal  to  free  it  of  lumps  of  shale,  pyrite, 
sticks  of  wood,  bits  of  iron,  and  other  impurities.  The  result 
of  this  screening  was  the  production  of  21  per  cent,  of  the  total 
amount  treated  as  clean  lump  pyrite  of  l1/^  in.  minimum  size,  and 
of  an  analyzed  purity  which  in  all  the  tests  ran  more  than  40 
per  cent,  sulphur  and  in  some  as  high  as  45.4  per  cent.  By  this 
simple  process  of  crushing  and  screening,  it  was  possible  to  pro- 
duce 37.6  per  cent,  of  the  pyrite  immediately  in  the  form  of  a 
clean  marketable  product. 

The  material  passing  through  the  l1/^  in.  holes  of  the  disinte- 
grating screen  entered  a  small  trommel  or  revolving  screen  hav- 
ing a  screen  plate  with  holes  about  %  in.  in  diameter.  The  pur- 
pose was  to  separate  the  material  smaller  than  1%  in.  into  two 
sizes,  one  of  which  should  contain  all  sizes  between  B£  in.  and 
34  in.,  and  the  other,  all  sizes  below  }4  in.  If  desired,  the  same 
results  could  be  obtained  by  adding  an  outer  screen  plate  with 


PYRITE  RECOVERY  331 

V±  in.  round  holes  to  the  disintegrating  screen,  that  is,  by  making 
it  a  compound  concentric  screen.  It  is  probably  more  satisfac- 
tory to  use  separate  screens,  especially  if  the  matter  of  making 
repairs  easily  is  considered.  Where  all  the  sizes  less  than  l1/^  in. 
in  diameter  were  washed  or  jigged  together,  the  separation  of 
the  pyrite  from  the  coal  was  incomplete,  especially  in  the  fine 
sizes  below  about  a/4  in.  Jigs  are  not  well  adapted  for  the  treat- 
ment of  these  fine  sizes',  therefore  separate  treatment  of  the  ma- 
terial below  ^4  in.  should  be  made  on  a  special  concentrating  table 
designed  for  fine  material. 

Of  the  amount  falling  through  the  holes  of  the  disintegrating 
screen,  70  per  cent,  was  larger  than  /'/i  in.  This  material  larger 
than  */4  in.  in  diameter  and  smaller  than  l1/^  in.  was  sent  to  a 
two-compartment  Hartz  or  Liihrig  plunger  jig.  The  jig  used  in 
the  laboratory  is  of  the  two-compartment  commercial  type  and 
is  of  half  dimensions,  capable  in  every  way  of  giving  cbmmercial 
products.  From  this  jig  three  products  were  obtained.  No.  1 
was  clean  pyrite  product  from  the  first  compartment  draw-off 
which  amounted  to  22  per  cent,  of  the  total  feed  or  34.2  per  cent, 
of  the  total  pyrite  in  the  mineral.  The  sulphur  content  of  this 
product  ranged  from  42  per  cent,  to  46  per  cent.  No.  2  was 
material  from  the  second  compartment  amounting  to  7.8  per 
cent,  of  the  total  pyrite  or  2.3  per  cent,  of  the  amount  of  feed. 
This  material  was  a  true  middling  product ;  that  is,  it  consisted 
of  pieces  of  pyrite  and  coal  which  had  not  been  freed  from  each 
other.  In  other  words,  the  weight  of  any  piece  lodging  here 
was  not  quite  sufficient  to  cause  it  to  settle  in  the  first  compart- 
ment, and  still  the  piece  was  not  light  enough  to  allow  it  to  over- 
flow the  second  compartment.  No.  3  was  the  overflow  material 
from  the  second  compartment  which  was  found  to  be  practically 
clean  coal.  In  the  preliminary  runs  some  pieces  of  pyrite  were 
observed  in  this  product,  but  after  a  few  trials  to  get  the  correct 
adjustment  of  the  jig,  no  difficulty  was  experienced  in  obtaining 
a  coal  comparable  with  the  ordinary  screenings  furnished  by 
Illinois  coal  mines. 

Middlings  such  as  were  noted  in  the  second  compartment  were 
not  in  condition  to  be  marketed  since  their  sulphur  content  was 
only  27.4  per  cent.  In  commercial  practice,  if  the  quantity  of 


332  COAL  WASHING 

these  middlings  is  sufficient  to  warrant  it,  more  nearly  complete 
separation  may  be  accomplished  by  recrushing  to  a  finer  size 
and  passing  again  through  the  disintegrating  screen. 

In  coal  washing  work  in  Illinois  little  attention  has  been  paid 
to  material  under  }4  in.,  largely  because  material  of  this  size 
usually  contains  an  excess  of  refuse  and  because  it  does  not 
readily  dry  out  or  free  itself  of  water.  In  pyrite  washing,  how- 
ever, conditions  are  different.  A  considerable  portion  (21  per 
cent.)  of  the  material  crushed  in  the  rock  breaker  will  be  found 
to  be  under  a/4  in.  in  size.  Since  this  material  contains  about 
42.2  per  cent,  of  pyrite  and  since  this  fine  pyrite  has  become 
more  valuable  than  the  larger  sizes,  some  form  of  modern  ore- 
concentrating  table  should  be  used  to  separate  pyrite  from  coal 
in  these  sizes.  No  difficulty  will  be  found  in  freeing  these  sizes 
of  pyrite  of  water.  In  the  experimental  work,  a  laboratory 
concentrating  table  of  half  commercial  dimensions  was  used. 
The  material  fed  to  the  table  in  the  test  runs  was  effectively  sepa- 
rated into  fine  pyrite,  containing  on  the  average  run  more  than 
40  per  cent,  sulphur,  and  fine  coal  which  might  be  added  to  the 
coal  obtained  from  the  jigs.  As  a  rule,  the  handling  of  quanti- 
ties of  such  fine  coal  presents  some  difficulty  because  of  the 
problem  of  removing  the  water  from  it  after  washing. 

Losses.  The  tests  indicate  either  81  or  87  per  cent,  recovery 
of  the  pyrite  as  shown  by  sulphur  analysis.  They  therefore  show 
a  loss  of  19  and  13  per  cent.,  respectively.  This  seems  to  be  a 
satisfactory  metallurgical  recovery  for  such  a  washing  process, 
especially  since  the  effort  has  been  to  employ  the  simplest  sort  of 
machinery. 

Probably  5  to  20  per  cent,  of  the  coal  in  the  smaller  sizes  will 
be  lost  during  the  process,  largely  by  passing  off  as  sludge  in  the 
water.  Analyses  indicate  that  this  sludge  is  too  impure  to  be 
used  as  coal.  Not  only  is  it  very  fine,  but  much  of  the  clay  or 
shale  impurity  which  is  intermixed  with,  the  coal  has  softened 
under  the  influence  of  the  water  and  passes  off  with  the  sludge. 

Water  Supply.  In  any  wet  concentration  process  using  jigs 
and  concentrating  tables,  the  question  of  an  adequate  supply  of 
water  is  important.  Water  is  used  in  spraying  the  rock  in  the 
breaker  for  the  purpose  of  keeping  down  dust ;  it  is  used  in  the 
disintegrating  screen  to  assist  in  the  cleaning;  and  from  this 


PYRITE  RECOVERY  333 

point  the  material  under  1H  in.  in  size  is  practically  flowing  in  a 
stream  of  water.  The  jig  and  concentrating  table  both  require 
water,  for  the  feed,  for  the  separating  process,  and  for  carrying 
away  the  separated  products.  By  the  use  of  perforated  elevators 
and  draining  bins  it  is  possible  to  recover  all  the  water  draining 
from  the  products  in  a  central  pond  or  sump  and  to  use  it  over 
and  over  by  pumping.  The  sediment  or  sludge  in  the  water  con- 
sists largely  of  fine  coal  and  clay,  since  the  pyrite  is  too  heavy 
to  pass  off  with  the  water  except  in  the  smallest  sizes.  The  set- 
tling pond  or  sump  common  at  Illinois  coal  washeries  can  be  re- 
placed to  good  advantage  by  a  large  round  settling  tank  20  or 
30  ft.  in  diameter  equipped  with  a  uniformly  horizontal  rim  over 
which  the  water  may  flow. 

The  sludge  water  from  the  plant  should  enter  this  tank  at  the 
center  and  under  the  surface  of  the  water.  Passing  toward  the 
rim  of  the  tank,  the  sludge  will  settle  to  the  bottom  and  the  water, 
sufficiently  clean  to  be  reused,  will  overflow  the  rim  and  may  be 
directed  into  a  small  sump  from  which  it  may  be  pumped  back  to 
the  plant.  The  tank  should  be  equipped  with  a  steeply  sloping 
bottom  so  that  the  accumulating  sludge  may  be  easily  removed. 
A  settling  tank  of  similar  character,  which  has  been  used  for 
many  industrial  purposes,  is  the  Dor  settling  tank.  The  im- 
portance of  fairly  clean  water  in  the  operation  of  a  plant  of  this 
kind  may  be  readily  understood  since  water  used  several  times 
without  settlement  of  the  sludge  often  contains  as  much  as  3  per 
cent,  of  solids  in  suspension.  Such  water,  draining  from  the 
washed  pyrite,  will  contaminate  it  by  depositing  solid  material 
on  its  surface  and  will  thus  lower  the  sulphur  percentage  of  a 
product  otherwise  satisfactory. 

In  cooperation  with  Wilbur  A.  Nelson,  E.  A.  Holbrook  made 
further  tests,  under  the  direction  of  the  Bureau  of  Mines,  to 
determine  the  value  of  pyrite  found  in  Tennessee  x  coals.  The 
report  on  these  tests  outlines  the  known  coal  pyrite  resources  of 
Tennessee  and  details  tests  made  at  Urbana,  111.,  on  crude  pyrite 
from  the  mines  of  the  Bon  Air  Coal  and  Iron  Corporation,  to 
learn  if  the  material  could  be  mechanically  treated  to  produce  a 
pyrite  of  commercial  purity. 

i  "Coal  Pyrite  Resources  of  Tennessee,"  by  E.  A.  Holbrook  and  Wilbur  A. 
Xelson.  Coal  Age  (Vol.  15,  Xo.  24,  1919). 


334  COAL  WASHING 

Outside  of  the  large  deposits  of  pyrite  and  pyrrhotite  in  east 
Tennessee  in  the  Ducktown  region,  the  state  has  an  additional 
source  of  pyrite  from,  certain  of  the  coal  seams  of  the  Cumber- 
land Plateau. 

The  mines  in  the  Bon  Air-Clifty  district  all  contain  pyrite  in 
the  form  of  bands,  nodules  and  kidneys,  which  are  easily  sepa- 
rated from  the  coal  and  can  be  recovered  as  a  byproduct.  It  is 
estimated  that  the  amount  of  pyrite,  if  all  is  recovered,  from  the 

Original  Material,  422  Ib.     32.68%  Sulphur 
Trial  Test 

Crushed  to  %-in.  Maximum  Size 
Jigged  Without  Sizing  on  a  2-compartment  Jig 

1st  Screen  Bed    1st  Hutch         2d  Bed  2d  Hutch  Tailings  Loss 

Concentrates  Concentrates  Middlings  Concentrates       ( Coal )  34  Ib. 

156  Ib.              24  Ib.  119  Ib.  26.0  Ib.  63.0  Ib.              8.1% 

37.0%               5.7%  28.2%  6.1%  '  14.9%                

43.1%S.         44.0%S.  36.7%S.  36.1%S.           5.3%S. 

•  •  , 19.8%  Ash 


Recrushed  to  %-in.  size 
Butchart  Concentrating  Table 


Concentrates 
85  Ib. 

71.4% 
41.6%S. 

1 
Middlings 
81b. 
6.7% 
30.1%S. 

Tail 
1911 
16.0 

ings 
). 
% 
%ft 

Loss 
71b. 

5.9% 

32.1%  Ash 

NOTES — 1st  Screen  Bed  Concentrates  means  the  coarse  concentrates  saved  on  the 
1st  bed  of  the  jig. 

2d  Hutch  Concentrates  means  the  fine  concentrates  passing  through  this  screen 
and  saved  at  the  bottom  of  the  jig. 

Middlings  means  a  product  of  pieces  containing  part  coal  and  part  pyrite  which  have 
to  be  crushed  finer  before  any  separation  of  clean  pyrite  can  be  made. 

FIG.   181.     FLOW  SHEET  or  COAL  PYRITE  CONCENTRATION  MATERIAL  FROM 

EASTLAND  MINE 

mines  in  this  district  when  operating  at  full  capacity  will  be 
at  least  50  tons  daily.  This  estimate  was  made  to  include  all  the 
mines  operating  on  the  Bon  Air  branch  of  the  Nashville,  Chatta- 
nooga &  St.  Louis  Railway. 

Clean  samples  of  pyrite  from  some  of  these  mines  gave  the 
following  analyses: 


PYRITE  RECOVERY  335 


Carola  Shaft,   Bon  Air,  Tenn 47.0  per  cent,  sulphur 

Braeburn   Mine,    Eastland,    Tenn 47.6  per  cent,  sulphur 

Ravenscroft  Mine,  Ravenscroft,   Tenn 46.4  per  cent,  sulphur 

The  mines  on  the  Monterey  branch  of  the  Tennessee  Central 
contain  pyrite  in  a  recoverable  form.  This  pyrite  is  similar  to 
that  from  the  Bon  Air  district,  on  which  tests  were  made.  It  is 
estimated  that  probably  40  tons  of  pyrite  a  day  could  be  recov- 
ered from  this  district.  Clean  samples  of  pyrite  were  taken 
from  some  of  these  mines,  which  gave  the  following  analyses : 

Fentress  Coal  Co.,  Wilder,  Tenn 46.4  per  cent,  sulphur 

Peacock   Mine,   Big  Mountain   Coal   Co.,   Obey   City, 

Tenn.,  Weather  surface  pyrite    46.1  per  cent,  sulphur 

Hand  cobbed  samples  of  pyrite  were  taken  from  these  mines, 
so  as  to  show  the  approximate  percentage  of  sulphur  in  carload 
lots  of  unwashed  pyrite  as  it  would  be  shipped  from  the  mines  if 
no  plant  for  treating  the  pyrite  was  installed.  The  following 
analyses  show  these  results : 

Carola  Shaft,  Bon  Air,  Tenn 43.88  per  cent,  sulphur 

Braeburn  Mine,  Eastland,  Tenn 46.40  per  cent,  sulphur 

Ravenscroft  Mine,   Ravenscroft,  Tenn 43.72  per  cent,  sulphur 

Clifty  Mines,  Clifty,  Tenn 42.72  per  cent,  sulphur 

Fentress  Coal  Co.,  Wilder,  Tenn 45.08  per  cent,  sulphur 

Peacock  Mine,  Big  Mountain  Coal  Co.,  Obey  City, 

Tenn ! 40.36  per  cent,  sulphur 

Brier  Hill  Collieries,  Crawford,  Tenn 36.08  per  cent,  sulphur 

The  pyrite  from  the  Fentress  Coal  Co.  has  been  shipped  to  an 
acid  manufacturer  for  some  time,  with  satisfactory  results  to 
both  the  mine  owners  and  the  acid  makers.  The  pyrite  from 
all  these  mines,  with  the  probable  exception  of  the  Brier  Hill 
collieries,  would  be  satisfactory  for  acid  making  after  having 
been  crushed  and  cleaned. 

Pyrite  is  also  found  in  a  few  of  the  mines  in  the  Tracy  City- 
Coalmont  district,  but  not  in  sufficient  quantities  to  justify  saV- 
ing  and  shipping.  In  the  northeastern  Tennessee  coal  field  some 
pyrite  occurs  but  no  detailed  investigation  was  made  of  this  area. 

About  Sept.  1,  1918,  two  shipments  of  crude  coal  pyrite  were 
received  at  the  laboratory  of  the  U.  S.  Bureau  of  Mines,  Univer- 
sity of  Illinois,  Urbana,  111.,  from  the  Eastland  and  Ravenscroft 


336  COAL  WASHING 

Original  Material,  809  Ib.     Sulphur  Analysis,  40.5% 
Crush  to  %-in.  size 


1st  Bed 
Pyrite 
2141b. 

*26.5% 
45.2%S. 

1                          I 
1st  Hutch           2d  Bed 
Pyrite           Middlings 
38  Ib.               268  Ib. 
4.7%                33.2% 
44.0%S.          43.9%S. 

2d  Hutch 
Middlings- 
58  Ib. 

7.2% 
43.4%S. 

Waste 
Tailings 
173  Ib. 
21.4% 
26.3%S. 
45.3%A. 

Loss 
57  Ib. 

7.0% 

Crush  to  ^-in.  size 
308  Ib. 
Treat  on  a  Concentrating  Table 

! 

1 

Clean  Pyrite 
244  Ib. 

75.0% 
44.9%S. 

1 
Middlings 
25  Ib. 

7.7% 
40.5%S. 

j 
Tailings 
37  Ib. 

11.3% 
38.1%S, 
52.0%  A. 

Loss 
2.0  Ib. 

6.0% 

*  26.5  per  cent,  means  that  26.5  per  cent,  by  weight  of  the  original  material  was 
saved  here. 

FIG.  182.    MINE  REFUSE  FROM  RAVENSCROFT  OPERATION  OF  BON  AIR  COAL 
AND  IRON  CORPORATION 


mines  of  the  Bon  Air  Coal  and  Iron  Corporation,  of  Bon  Air, 
Tenn.  The  possibilities  of  utilization  of  coal  pyrite  on  a  large 
scale  for  the  manufacture  of  sulphuric  acid  made  it  desirable  to 
conduct  tests  on  this  material  to  learn  if  mechanical  crushing 
and  washing  would  produce  a  high-grade  commercial  pyrite  free 
from  coal  and  other  impurities  and  with  possibly  clean  coal  as  a 
byproduct.  The  report  following  gives  an  outline  of  the  final 
tests,  together  with  a  flowsheet  outlining  a  possible  method  of 
mechanically  treating  these  materials. 

This  lot,  about  500  Ib.  of  crude  coal  pyrite  marked  from  the 
Eastland  mine,  consisted  of  lenses  of  pyrite  up  to  10  in.  in  width 
and  4  in.  in  thickness,  together  with  considerable  adhering  coal. 
To  the  eye,  about  50  per  cent,  of  the  lumps  by  volume  was  pyrite 
and  the  remainder  was  coal. 

Preliminary  tests  showed  that  the  clean  pyrite  in  the  material 
was  high  grade,  and  that  crushing  to  about  1-in.  size  would  pro- 
duce a  clean  pyrite  concentrate,  and  consequently  fairly  clean 


PYRITE  RECOVERY  337 

coal  as  a  byproduct.  During  crushing,  a  comparatively  small 
amount  of  pyrite  fines  was  produced,  and  therefore  the  largest 
sizes  were  the  richest  in  pyrite.  In  other  words,  this  coal  pyrite, 
unlike  the  usual  pyrite  mineral,  is  stony  and  amorphous  in 
structure  and  does  not  slime  on  crushing.  This  very  important 
point  made  it  possible  to  recover  most  of  the  pyrite  by  jigging 
alone,  and  the  concentrating  table  was  necessary  only  in  treating 
the  recrushed  middlings.  Even  this  product  might  be  fed  into 
the  jig  and  saved  as  a  hutch  product,  providing  the  capacity  of 
the  jig  was  ample. 

The  accompanying  quantity  flow-sheet  (Fig.  181)  shows  the 
results  of  the  final  test  run  on  the  pyrite  from  the  Eastland  mine. 
Four  hundred  and  twenty-two  pounds  were  crushed  to  %  in. 
si-ze  in  a  gyratory  crusher  followed  by  rolls.  This  was  jigged  in 
a  two-compartment  Hartz  jig  with  M  in.  screen  beds.  The  coarse 
concentrates  were  saved  as  a  screen-bed  product  and  the  fine  con- 

RESUME  OF  RUN  OF  EASTLAND  MINE,  BON  AIR  COAL  AND  IRON  CORPORATION 
Total,  422  lb.;  32.7  per  cent,  sulphur 


Product 

Concentrates             Middlings 
Analysis                  Analysis 
Sulphur,                  Sulphur, 

Tailings 
Coal 
Analysis 

Loss 

First      bed      concen- 
trates   

156 
24 

43  1 

First   hutch    concen- 
trates 

44  0 

Second       bed       mid- 
dlings    (  see    table 
products)    .... 

Second  hutch 

26         36  1 

63           5.38. 
19.  8A. 

Jio1  tailings    . 

41  6 

Table  concentrates   . 
Table  middlings    .  .  . 
Table  tailings 

85 

8         30.1 

i9          12  .  3S. 
32.1  A. 

34 

7 

41 

Jiff  loss 

Table  loss 

Totals    

265 

42.7         34         34.7 

82           6.9S. 
22.  6A. 

Practically  265  lb.  of  commercial  pyrite  was  recovered,  or  62.8  per  cent,  of  the 
total  material  treated.  On  further  treatment,  the  34  lb.  of  middlings  could  be  ex- 
pected to  yield  25  lb.  of  commercial  pyrite,  making  a  total  recovery  of  290  lb.  of 
pyrite  or  68.7  per  cent.  The  coal  tailings  were  82  lb.  or  19.4  per  cent.  The  treat- 
ment loss  was  41  lb.,  or  9.7  per  cent. 


TABLE  53 


338  COAL  WASHING 

centrates  were  saved  as  a  hutch  product.  The  second  screen-bed 
product  contained  some  coal  adhering  to  the  pyrite  and  was  a 
true  middling  product.  It  was  therefore  crushed  through  a 
}4  in.  screen  and  treated  on  a  Butchart  concentrating  table.  The 
table  cleaned  this  product  and  produced  a  high-grade  concen- 
trate. 

The  crude  coal  pyrite  from  the  Ravenscroft  mine  was  of  about 
the  same  physical  appearance  as  the  material  from  the  Eastland 
mine.  Some  of  the  lenses,  however,  were  of  rather  light  weight 
and  had  a  peculiar  gray  color.  To  the  eye  it  appeared  about  75 
per  cent,  pyrite  by  volume  while  the  remainder  was  adhering 
coal.  This  material  was  tested  in  a  preliminary  way  and  the 
tests  indicated  that  the  same  treatment  could  be  used  as  with  the 
material  from  the  Eastland  mine.  This  is  a  point  of  importance, 
because  in  any  concentrating  plant  it  would  allow  indiscriminate 
mixing  of  the  material  from  the  various  mines  before  treatment. 
On  crushing,  the  crude  pyrite  produced  only  a  small  percentage 
of  fines,  and  it  appeared  that  in  regular  practice,  crushing  to 
1  in.  round  hole  size  would  be  sufficient  before  attempting  con- 
centration. 


CHAPTER  XXXII 
WATER  SYSTEM 

In  a  washery  the  jigs  use  most  of  the  water  required,  but  de- 
pending upon  the  different  installations  water  is  also  used  for 
spraying,  in  dust  collectors,  and  in  sludge  treatment.  In  a 
general  way  it  can  be  assumed  that  about  from  three  to  six  tons 
of  water  are  required  for  each  ton  of  coal,  or  from  725  to  1450 
gal.  of  water  must  be  put  in  circulation  for  each  ton  of  coal 
treated.  But  the  amount  of  water  actually  necessary  varies  a 
great  deal  with  the  character  of  the  raw  coal,  the  number  of  sizes 
made  and  the  expected  output.  The  last  point  demands  especial 
consideration. 

The  water  consumption  increases  immensely  if  the  washery  is 
overloaded.  In  such  cases  the  water  must  assume  part  of  the 
work  which  the  overloaded  jigs  cannot  perform  to  the  required 
degree  of  exactness.  Table  54  shows  what  quantities  of  water 


Size    of    Coal 

Per- 
centage 

Amount 
in 
Tons 

Water 
Required 
in  Gallons 
per  Ton 
of  Coal 

Water 
Required 
in  Gallons 
per    Day 

Lump  coal   

20 

600 

Nut  coal,  %  to  3  in..  . 
Fine  coal,  %  to  %  in.  .  . 
Sludge,  0  to  %  in  
Fresh  water  for  spray- 
ing nut  coal  

35 
30 

15 

1,050 
900 
450 

965 
1,440 
240 

24 

1,013,250 
1,296,000 
108,000 

25,200 

Total 

100 

3,000 

2,442,450 

TABLE  54 

are  required  in  a  washery.  We  assume  a  mine  hoisting  3000 
tons  of  coal  per  day  and  that  80  per  cent,  of  this  amount  will 
be  handled  in  the  washery.  The  table  shows  the  different  sizes 
of  the  washed  coal  made  and  the  required  quantities  of  water. 
If  the  washery  is  designed  for  a  daily  capacity  of  2400  tons — 

339 


340  COAL  WAS  HIS  C 

that  is,  for  a  12-hour  shift — the  hourly  water  requirements  are 
203,537  gal.  or  1017  gal.  of  water  per  ton  of  coal. 

It  thus  becomes  clear  that  only  in  extremely  exceptional  cases 
can  the  clarification  and  reuse  of  the  wash  water  be  neglected. 
Assuming  the  cost  of  water  at  only  0.005  of  one  cent  per  gallon 
(which  means  20,000  gal.  for  $1),  the  water  alone  would  cost 
5.09c.  per  ton  of  coal  and  the  daily  expenditure  for  a  washery 
with  an  output  of  2400  tons  of  coal  would  be  $122.16  for  water 
alone.  Therefore,  every  effort  should  be  made  to  clarify  and 
recirculate  the  water  without  appreciable  wastage. 

Water  losses  can  be  divided  into  unavoidable  and  avoidable 
ones.  Unavoidable  ones  are  brought  about  by  evaporation  and 
by  a  certain  amount  of  water  being  carried  away  with  the  washed 
coal,  the  refuse  and  the  sludge.  These  losses  are  increased  by 
any  necessity  for  rapid  operation,  which  gives  little  time  for 
drainage.  Only  in  the  bins  has  the  coal  time  to  lose  some  of  the 
water.  With  the  installation  of  mechanical  dryers,  however,  this 
loss  has  been  greatly  diminished  as  most  of  the  water  adhering 
to  the  coal  is  returned  to  the  system.  But  there  still  remains  the 
loss  of  water  caused  by  the  moisture  in  the  outgoing  refuse  and 
sludge. 

The  loss  of  water  that  drains  out  of  the  bins  is  avoidable 
through  collecting  it  in  gutters.  Avoidable  also  are  the  losses 
caused  by  leaky  tanks  and  sluice-ways.  These  losses  increase 
with  the  age  of  the  washery  and  can  hardly  be  entirely  elimi- 
nated. The  use  of  steel,  cast  iron  and  concrete  for  tanks  and 
sluiceways  will  cut  down  this  loss  considerably  and  will  also 
make  the  whole  plant  a  good  deal  cleaner.  The  idea  that  a 
washery  must  be  sloppy  is  not  only  erroneous,  but  expensive. 

The  amount  of  the  water  losses  varies  widely  with  the  con- 
struction of  the  washery,  its  age  and  the  materials  used  in  its 
construction.  It  is  safe  to  assume  such  loss  as  amounting  to  from 
8  to  10  per  cent,  of  the  total  quantity  used.  This  amount  must 
be  taken  into  consideration  in  figuring  upon  the  necessary  fresh- 
water supply.  Whether  these  figures  will  be  sufficient  depends 
entirely  upon  the  efficiency  of  the  water-clarification  plant. 

If  mine  water  which  is  acidulous  or  salty  is  used,  greater 
quantities  must  be  wasted  so  as  not  to  increase  the  acidity  of  the 
water  beyond  a  safe  point.  If  concrete  is  largely  used  in  the 


WATER  SYSTEM  341 

construction  of  tanks  and  sluiceways,  care  must  be  taken  to  keep 
the  acidity  of  the  water  within  close  limits,  as  acid  water  has  a 
disastrous  effect  upon  concrete  structures. 

In  general  the  degree  of  water  clarification  desirable  depends 
upon  the  proportionate  cost  of  power  and  water,  the  possibility 
of  clarifying  the  water  and  of  allowing  the  dirty  water  to  run 
away  without  damaging  adjoining  property  or  polluting  streams. 

For  water  circulation  in  the  washery  centrifugal  pumps  are 
almost  universally  used.  The  character  of  the  water,  the  re- 
quirement of  lifting  large  volumes  of  water  under  compara- 
tively low  heads  and  the  floor  space  at  disposal  forbidding  large 
pumprooms,  render  centrifugal  pumps  especially  advisable.  It 
must  be  emphasized  also  that  the  whole  washer  operation  depends 
upon  the  uninterrupted  service  of  the  circulating  pumps ;  there- 
fore, it  would  be  mistaken  economy  to  leave  a  spare  circulating 
pump  out  of  the  washery  equipment  merely  on  account  of  lack 
of  convenient  space  or  a  shortage  of  money. 

The  fact  that  water  clarification  is  the  final  process  places  the 
pump  cistern  at  the  lowest  point  of  the  washery.  It  is  important 
to  make  the  pump  cistern  big  enough  to  take  care  of  all  the  water 
in  circulation  when  the  pumps  are  shut  down  and,  on  the  other 
hand,  to  give  the  pumps  sufficient  water  from  which  to  draw  at 
the  beginning  of  the  operation.  It  has  been  found  advisable  to 
interpose  between  the  circulating  pump  and  the  jigs  a  water 
tank  or  high-level  reservoir  for  the  purpose  of  supplying  the 
jigs  with  water  under  constant  pressure  and  at  the  same  time 
to  provide  further  storage  space. 

The  power  required  for  the  circulating  pumps  varies  consider- 
ably, depending  upon  the  volume  of  water  to  be  circulated  and 
upon  the  difference  in  elevation  between  the  pump  cistern  and 
the  jig  tanks.  Approximately,  it  can  be  assumed  that  for  a 
washer  having  a  capacity  of  100  tons  per  hour  there  are  re- 
quired 70  to  125  h.p. ;  for  150  tons  per  hour,  100  to  150  h.p.; 
for  200  tons  per  hour,  140  to  170  h.p. ;  for  250  tons  per  hour, 
160  to  250  h.p.  Besides  the  circulating  pumps  several  other 
pumps  are  required  to  handle  the  sludge  from  the  thickeners  and 
the  clear  water  and  the  sludge  from  the  clearing  basins.  It  is 
also  advisable  to  install  a  high-pressure  pump  for  fire  protection 
and  for  the  purpose  of  washing  off  the  floors. 


CHAPTER  XXXIII 
POWER 

The  amount  of  power  required  depends  primarily  upon  the 
capacity  of  the  washery.  The  following  must  be  considered  to 
determine  the  total  amount  of  power  required :  The  methods  of 
operating  the  screens,  the  jigs,  the  dust  collectors,  the  crushers, 
etc. ;  in  short,  all  of  the  mechanically  operated  equipment.  This 
in  turn  depends  upon  the  character  of  the  raw  coal  and  its  im- 
purities. The  power  required  for  each  piece  of  apparatus  de- 
signed for  a  certain  capacity  and  material  is  known;  therefore, 
the  summation  of  the  power  required  for  all  the  apparatus  gives 
the  total  power  necessary.  To  this  total,  however,  must  be  added 
a  certain  percentage  to  take  care  of  the  power  losses  sustained  in 
transmission. 

Local  conditions  and  arrangements  of  the  machinery  influence 
power  consumption.  To  reduce  the  power  requirements  to  a 
minimum  it  is  desirable  to  either  use  the  natural  elevation  or  to 
raise  the  raw  coal  to  such  a  height  that  the  flow  of  the  materials 
can  be  carried  on  by  gravity  alone  or  with  the  aid  of  sluicing 
water.  In  a  level  country  there  are  some  limitations  to  this 
ideal  condition  on  account  of  the  difficulty  encountered  in  de- 
signing and  operating  heavy  elevators  of  great  capacity  in  an 
economical  manner. 

The  power  required  per  ton  of  coal  treated  will  vary  between 
considerable  limits.  Average  values  taken  from  existing  installa- 
tions are  given  as  from  2  to  3  h.p.  per  ton  of  hourly  capacity. 
Some  modern  installations,  however,  with  a  complete  system  of 
water  clarification  and  sludge  recovery,  require  as  much  as 
5  h.p.  per  ton  of  hourly  capacity. 

From  the  foregoing  discussion  it  can  easily  be  seen  that  only 
after  a  careful  examination  of  all  the  details  will  it  be  possible 
to  decide  upon  a  suitable  general  arrangement.  Furthermore, 
the  cost  of  power  plays  an  important  part  in  the  proper  selec- 

342 


POWER 


343 


tion  of  the  machinery.  A  mine  paying  only  $4c.  per  kilowatt- 
hour  can  consider  in  the  selection  of  the  machinery  other  advan- 
tages than  a  mine  paying  ll/2  cents. 

Table  55  gives  the  average  power  required  for  the  different 
pieces  of  apparatus  used : 


Description  of  Apparatus 


Power     Required     for     a     Washery 
Having   a   Capacity  per   Hour  of 

, Ton  s 

100  150  200 


1. 

Dust  collector   in  screen  house  

5  to 

18 

6  to 

18 

7  to 

18 

2. 

Screens  in  tipple  

6  to 

15 

8  to 

25 

15  to 

40 

3. 

Picking  tables  and  loading  booms.  .  .  . 

10  to 

15 

10  to 

25 

15  to 

30 

4. 

Conveying  rock  and  picked-out  slate.  . 

6  to 

15 

6  to 

15 

6  to 

15 

5. 

Conveyors  from  screen  to  fine  coal  bin 

5  to 

10 

6  to 

12 

8  to 

15 

6. 

Crushers   

80  to 

120 

100  to 

160 

150  to  200 

7. 

Raw  coal  elevator  

15  to 

30 

20  to 

50 

30  to 

60 

8. 

Conveyors  for  raw  coal  storage  bin  .  . 

5  to 

10 

5  to 

12 

5  to 

15 

9. 

Magnetic  separator    

5  to 

10 

5  to 

10 

7  to 

15 

10. 

Preliminary  screens   

5  to 

10 

7  to 

15 

10  to 

20 

11. 

Dust   collector    

5  to 

10 

5  to 

15 

6  to 

20 

12. 

Coarse  coal  jigs  

15  to 

25 

20  to 

40 

40  to 

52 

13. 

Coarse  refuse   elevators  

5  to 

10 

7  to 

12 

10  to 

15 

14. 

Rescreening  of  nut  coal  

5  to 

8 

5  to 

12 

7  to 

15 

15. 

Conveying  nut  coal  to  storage  bins  .  .  . 

5  to 

6 

5  to 

8 

6  to 

10 

16. 
17. 

Conveying  middle  products  
Crushing  middle  products  

5  to 
10  to 

6 
30 

5  to 

20  to 

8 
40 

6  to 
30  to 

10 
60 

18. 

Rewash  jigs  

5  to 

10 

10  to 

15 

15  to 

20 

19. 

Fine  coal  jigs  

10  to 

15 

15  to 

20 

20  to 

30 

20. 

Concentrating  tables   

7  to 

12 

10  to 

15 

15  to 

20 

21. 

Fine  refuse  elevators  

2  to 

5 

3  to 

6 

5  to 

8 

22. 

Conveying  fine  coal  to  storage  bins.  . 

8  to 

20 

12  to 

30 

15  to 

30 

S3 

Drving  of  fine  coal  

60  to 

100 

100  to 

150 

150  to  200 

24. 

Sludge  recovery    

5  to 

10 

10  to 

15 

15  to 

20 

25. 

Water   circulation    

70  to 

125 

100  to 

150 

140  to  175 

TABLE  55 


CHAPTER  XXXIV 
ARRANGEMENT  OF  MOTORS  AND  DRIVES 

In  the  earlier  washeries  frequently  only  one  main-drive  unit 
(usually  a  steam  engine)  was  employed  for  the  whole  plant,  or 
one  engine  drove  the  washery  and  another  the  screening  plant. 
The  power  had  to  be  transmitted  from  one  point  to  all  the  differ- 
ent pieces  of  apparatus.  This  resulted  in  complicated  systems 
of  transmission  machinery  distributed  over  the  entire  plant. 
The  disadvantages  of  this  arrangement  were  well  known,  even  at 
that  time,  but  as  long  as  only  steam  was  available  as  the  sole 
source  of  power,  a  decentralization  of  the  power  supply  was  out 
of  the  question  on  account  of  the  great  weight  and  large  size  of 
the  steam  engines. 

The  disadvantages  of  such  a  centralized  power  station  are  as 
follows:  The  great  number  of  shafts,  pulleys,  belts,  sprocket 
wheels,  chains,  sheaves,  ropes  and  clutches  makes  the  installation 
.expensive  in  first  cost  as  well  as  in  cost  of  operation.  The  super- 
vision of  such  a  plant  is  difficult,  costly  and  dangerous.  It  re- 
quires a  large  crew  to  attend -to  the  lubrication  and  upkeep  of 
all  this  complicated  machinery.  The  loss  of  power  caused  by 
friction  and  inefficient  transmission  machinery  is  enormous. 
The  swiftly  moving  belts,  chains,  ropes  and  shafting  are  a  con- 
stant source  of  danger  to  the  operator.  The  necessary  safe- 
guards are  expensive  and  at  best  only  a  cumbersome  makeshift. 

It  is  consequently  only  quite  natural  that  the  direct  electric- 
motor  drive  has  been  quickly  adopted  for  coal  washeries.  This 
permits  the  installation  of  small  independent  drives,  avoiding 
all  cumoersome,  expensive  and  dangerous  transmission  machin- 
ery. The  small  motors  can  easily  be  placed  in  almost  any  posi- 
tion without  heavy  or  expensive  foundations. 

For  centrifugal  pumps  and  crushers  the  electric  motor  drive  is 
especially  well  adapted.  Electric  drives  permit  the  different 
units  to  be  operated  independently  one  from  the  other.  They 

344 


MOTORS  AXD  DRIVES  345 

can  be  stopped  easily  and  quickly  by  throwing  a  switch,  which 
enhances  the  safety  of  the  operation.  The  control  of  all  motors 
can  be  consolidated  on  a  central  switchboard,  so  that  by  using  a 
remote-control  system  any  unit  can  be  started  or  stopped  from  a 
central  point.  Furthermore,  cutout  switches  can  be  placed  at 
convenient  points  throughout  the  plant,  so  that  in  case  of  danger 
it  is  not  necessary  to  go  to  the  motor  or  the  central  control  board. 
Disastrous  and  costly  wrecks  can  thereby  be  avoided. 

The  starting  apparatus  of  the  different  machines  forming  one 
unit  can  be  connected  in  such  a  way  that  it  will  be  impossible 
to  start  one  machine  before  the  following  one  has  been  put  in 
operation  or,  vice  versa,  to  stop  a  machine  before  the  preceding 
one  has  been  shut  down.  This,  in  case  of  crushers,  elevators  and 
conveyors  .will  avoid  choking  up  any  piece  of  apparatus  and 
spilling  coal.  It  is  easy  to  make  the  operation  of  an  electrically 
driven  plant  foolproof  by  taking  the  successive  starting  of  the 
separate  pieces  of  machinery  out  of  the  hands  of  the  machine 
operator. 

The  starting  apparatus  should  be  provided  with  an  overload 
circuit  breaker  so  that  in  case  of  a  jam  in  the  machinery,  wrecks 
or  burnouts  of  the  motors  will  be  avoided.  No-voltage  releases 
ought  to  be  installed  also,  so  that  in  case  of  a  sudden  failure  of 
the  power  supply  the  motors  will  not  start  when  the  power  comes 
on  again.  It  should  be  possible  to  lock  the  starting  apparatus,  to 
provide  a  safeguard  for  the  men  repairing  the  machinery. 

Slow-speed  motors  are  in  most  cases  advisable  on  account  of 
the  extra  expense  and  increased  loss  of  power  caused  by  speed- 
reducing  gears.  Constant-speed  motors  with  a  good  starting 
torque  should  be  employed,  except  for  elevator  and  jig  drives 
where  a  variation  of  speed  is  sometimes  required.  Washed  coal 
and  refuse  elevator  drives  should  be  designed  to  permit  the  re- 
ducing of  the  elevator  speed  for  short  periods. 

The  only  disadvantage  of  electric-motor  drive  encountered  in 
actual  operation  arises  from  the  inability  to  change  the  speed 
within  the  limits  sometimes  required  in  the  operation  of  a  wash- 
ery.  It  is  necessary  to  slow  down  the  greater  part  of  the  ma- 
chinery at  certain  intervals  to  permit  a  careful  and  thorough  in- 
spection. For  this  purpose  the  speed  of  the  machinery  should 
be  reduced  at  least  to  25  per  cent,  of  the  normal  working  speed. 


346  COAL  WASHING 

With  steam  engines  as  main-drive  units  the  speed  of  the  machin- 
ery can  be  reduced  to  almost  any  degree  and  the  starting  and 
stopping  can  be  accomplished  without  exposing  the  machinery 
to  sudden  stresses  and  shocks. 

The  question  remains,  How  far  should  decentralization  be  car- 
ried? To  install  a  s'eparate.motor  for  each  piece  of  apparatus 
would  require  an  undesirable  number  of  small  motors,  which 
would  increase  the  cost  of  installation  out  of  all  proportion  to 
the  advantages  gained  thereby.  The  whole  electrical  equipment 
would  become  complicated  and  the  control  unwieldy. 

It  will  be  far  more  advisable  to  combine  the  drives  for  a  group 
of  machinery,  making  thus  one  drive  unit,  if  one  motor  can 
actuate  it  by  means  of  simple,  conveniently  arranged  transmis- 
sion apparatus.  This  is  especially  the  case  with  jig  drives. 
Therefore,  we  must  consider  in  the  selection  of  a  proper  drive  the 
following:  The  degree  of  decentralization  depends  upon  the 
space  at  disposal.  This  sometimes  requires  a  fixed  arrangement 
of  the  machinery,  regardless  of  the  convenient  arrangement  of 
the  drives.  In  some  cases,  however,  it  will  be  possible  to  con- 
sider the  most  convenient  and  economic  drives,  regardless  of 
other  requirements.  Therefore,  generally  speaking,  no  special 
method  of  driving  can  be  pronounced  as  the  best.  Each  sepa- 
rate case  demands  its  particular  solution  and  the  number  of 
motors  to  be  installed  will  vary  from  6  to  37.  In  the  simplest 
case  the  motors  can  be  arranged  into  groups  as  follows:  (1) 
Raw-coal  elevator  and  preliminary  screening;  (2)  all  the  jigs, 
the  washed  coal  and  refuse  elevators;  (3)  sizing  screens;  (4) 
washed  coal  conveyors  to  the  bins;  (5)  circulating  pump;  (6) 
sludge-handling  and  water  clarification. 

In  the  most  complicated  case,  where  the  decentralization  has 
been  carried  to  extremes,  we  find  the  following:  (1)  Docking 
table;  (2)  coal  conveyors  to  crusher;  (3)  feeders  under  unload- 
ing hopper;  (4)  cross  conveyor  from  unloading  hopper ;  (5)  con- 
veyor for  foreign  coal  to  crusher;  (6  and  7)  crushers;  (8)  con- 
veyor to  raw-coal  storage  bin;  (9)  reclaiming  conveyor  under 
storage  bin;  (10)  conveyor  to  screen  house;  (11  and  12)  sizing 
screens;  (13,  14  and  15)  conveyor  for  sized  coal  to  equalizing 
bins;  (16  and  17)  jigs;  (18  and  19)  washed-coal  elevator;  (20) 
refuse  elevator;  (21,  22,  23  and  24)  dryers;  (25)  washed-coal 


MOTORS  AND  DRIVES  347 

conveyor;  (26  and  27)  circulating  pumps;  (28,  29  and  30)  sludge 
pumps;  (31,  32,  33,  34  and  35)  thickeners;  (36)  concentrating 
tables;  (37)  laboratory  crusher. 

The  horsepower  of  the  foregoing  37  motors  varies  from  7^  to 
250,  and  two  voltages  are  used — that  is,  440  and  2300 — besides 
the  lighting  circuit  of  110  volts. 


CHAPTER  XXXV 
BUILDINGS  AND  STRUCTURES 

Timber  construction  is  rather  antiquated  and  undesirable  on 
account  of  the  fire  risk.  Only  in  certain  cases,  where  the  acreage 
of  the  mine  will  not  promise  a  long  life  will  it  be  excusable  to 
use  timber  in  the  construction  of  a  washery.  But  even  then  the 
danger  of  fires  must  be  considered.  Such  fires,  even  when  the 
washery  is  fully  insured,  entail  a  lengthy  interruption  to  oper- 


Fig.  183.     Timber  Work  of  Coal  Washery  in  Course  of  Construction 

ation  and  a  consequent  loss  of  profit,  or  even  the  loss  of  a  de- 
sirable customer. 

In  addition  to  this,  timber  construction,  on  account  of  the 
larger  size  of  timbers  necessary,  narrows  down  the  space  at  dis- 
posal and  the  great  number  of  joists,  beams  and  braces  interferes 
with  the  passageways  and  the  convenient  supervision  of  the 
plant.  Reinforced  concrete  for  the  building  proper  is  expen- 
sive and  has  the  further  disadvantage  that  changes  and  addi- 

348 


BUILDINGS  AND  STRUCTURES 


349 


tions  cannot  be  made  except  at  great  cost  and  under  difficulties. 
For  tanks,  sluiceways  and  bins,  reinforced  concrete  is  supreme. 
In  connection  with  this  it  may  be  stated  that  concrete  sluiceways 
ought  always  to  be  lined  with  glazed  terra-cotta  tile  to  resist 
abrasion.  For  the  construction  of  the  housing  over  the  machin- 
ery, steel  is  the  only  feasible  material.  A  steel  structure  makes 
a  light,  rigid  and  durable  building,  permitting  the  location  of 
plenty  of  windows  and  ventilators.  Daylight  is  the  cheapest 
item  we  have  at  our  disposal,  and  it  should  be  used  freely.  Ma- 


Fig.  184.     Photograph  Showing  Studdings  for  Side  Walls  of  Washery 

chinery  supports  can  be  arranged  easily,  and  floor  beams,  stair- 
ways and  walks  conveniently  placed  to  provide  accessibility  to 
all  parts  without  obstructing  the  view. 

For  the  covering  of  the  buildings  we  have  a  great  variety  of 
materials,  so  that  the  proper  selection  will  depend  upon  the  cli- 
mate, the  money  available  and  the  personal  preference  of  the 
designer.  Under  ordinary  conditions  galvanized  corrugated 
steel  sheets  are  quite  suitable  for  the  sides  of  the  buildings.  If 
painted  and  kept  in  good  repair,  they  will  last  a  reasonable  time ; 
but  even  under  the  most  favorable  conditions  the  cost  of  upkeep 


350 


COAL  WASHING 


is  considerable,  and  they  do  not  give  sufficient  protection  in 
colder  climates.  The  increased  cost  for  heating  may  easily  over- 
balance the  cheapness  of  corrugated  steel  siding. 

In  a  warm  climate  the  sides  can  be  arranged  in  sliding  panels 
so  as  to  give  plenty  of  fresh  air  in  the  summertime.  In  colder 
climates,  and  for  durability,  concrete  stucco  work  on  an  expanded 
metal  base  is  advisable.  This  offers  good  protection  against  the 
weather  and  does  not  require  painting  or  frequent  repairs.  It 
ought  to  last  as  long  as  the  steel  framework. 

Roofs  can  also  be  covered  with  galvanized  corrugated  steel 


Fig.  185.     Washery  Built  Entirely  of  Timber 

sheets,  but  asbestos  cement  in  the  shape  of  shingles  or  corrugated 
sheets  is  far  more  advisable.  Floors  should  be  made  of  rein- 
forced concrete  with  a  non-dusting  top  dressing  and  arranged 
in  such  a  way  that  they  can  be  easily  and  thoroughly  washed  off. 
Stair  treads  should  either  be  filled  in  with  concrete  or  made  of 
some  non-slip  material.  The  inside  of  the  building,  especially 
the  under  side  of  the  roofs,  should  be  painted  in,  say,  a  light 
gray  color. 

The  idea  that  a  washery  must  be  a  dark,  sloppy  place  has  long 
ago  been  exploded.  A  coal  washery  can  be  made  just  as  clean 
and  light  as  any  other  industrial  building.  Plenty  of  light  not 
only  means  convenience  but  also  safety.  Dark  corners  are  ta- 
booed in  modern  construction.  The  ideal  design  should  permit 
the  unobstructed  supervision  of  all  machinery  from  one  point. 


BUILDINGS  AND  STRUCTURES 


351 


352 


COAL  WASHING 


BL'1LDI.\(1H  .\\D  STRUCTURES  353 

The  main  requirements  to  be  considered  in  the  design  of  a  wash- 
ery  building  may  be  condensed  as  follows:  The  building  must 
give  sufficient  protection  against  the  inclemencies  of  the  weather ; 
all  vibration  must  be  taken  care  of ;  all  the  machinery  must  be  in 
full  and  unobstructed  view  from  preferably  one  but  in  any 
case  as  few  points  as  possible;  all  machinery  must  be  safely, 
fully  and  easily  accessible;  artificial  lighting  should  only  be  re- 
quired during  the  night-time;  no  dark  corners  should  be  per- 
mitted; changes  in  the  arrangement  of  the  machinery  must  be 
easily  accomplished. 

Figs.  183  and  184  show  the  framework  of  a  coal  washery  built 
entirely  of  timber.  Fig.  185  illustrates  the  same  washery  com- 
pleted. Figs.  186  and  187  are  photographs  of  a  modern  washery 
under  construction. 


CHAPTER  XXXVI 
COST  OF  WASHING  COAL 

A  guarantee  for  a  certain  amount  of  ash  in  the  washed  coal  is 
only  to  be  considered  if  at  the  same  time  a  certain  yield  is  also 
guaranteed.  The  cost  of  operation  is  an  important  factor.  An 
indisputable  guarantee  should  read :  With  x  cents  cost  of  oper- 
ation per  ton  of  such  and  such  a  coal  handled  we  guarantee  an 
output  of  y  per  cent,  with  z  per  cent,  of  ash  in  the  washed 
product. 

To  check  these  figures  it  is  necessary  to  take  average  samples 
of  the  different  products  and  analyze  them.  Therefore,  a  labora- 
tory is  a  necessary  appendage  to  a  washery.  Daily  samples 
ought  to  be  taken,  the  ash  and  sulphur  contents  determined,  also 
the  percentage  of  "sink"  in  the  washed  coal  and  the  percentage 
of  "float"  in  the  refuse.  These  results  ought  to  be  posted  on 
the  jig  floor  so  that  the  jig  runner  can  see  what  he  is  doing. 
To  do  this  with  any  degree  of  accuracy  it  is  imperative  to  install 
automatic  samplers,  described  on  pages  130  to  132. 

The  cost  of  operation  depends  upon  the  character  of 'the  raw 
coal,  just  as  the  yield  and  the  percentage  of  ash  and  sulphur  in 
the  washed  coal  depend  upon  it.  But  the  cost  of  operation  is 
furthermore  influenced  by  the  arrangement  of  the  washery  and 
the  supply  and  application  of  power  and  water.  General  condi- 
tions only  can  here  be  considered,  as  each  separate  case  must  be 
handled  in  a  different  way  and  individually.  Weekly  or  at  least 
monthly  cost  sheets  on  a  per  unit  (ton  of  input)  basis  are  of 
great  value,  especially  as  the  comparatively  simple  operation  of 
a  washery  permits  an  easy  and  correct  subdivision  of  the  cost 
for  all  separate  operations.  By  carefully  studying  and  compar- 
ing the  figures  obtained  valuable  information  can  be  gained  which 
will  be  a  guide  in  making  changes  in  the  method  of  operation. 
It  is  therefore  judicious  to  arrange  the  cost  sheets  according  to 

354 


COST  OF  WASHING  COAL  355 

the  different  units  of  operation,  so  that  we  get  the  cost  of  each 
step  of  the  process  separately. 

The  cost  of  operation  must  be  divided  into  fixed  charges,  op- 
erating expenses  and  the  cost  of  special  work.  It  is  only  natural 
to  keep  the  cost  of  installation  as  low  as  possible.  This  effort  in 
economy  is  limited,  however,  by  the  necessity  of  keeping  the 
cost  of  operation  and  that  of  repairs  as  low  as  possible.  If  one 
operator  can  be  saved  by  a  certain  increase  in  the  cost  of  in- 
stallation, this  increase  will  be  justified  if  it  is  lower  than  the 
capitalized  wages  of  the  operator.  This  is  because  it  is  desir- 
able to  become  as  far  as  possible  independent  of  the  imperfection 
of  human  labor. 

The  regular  cost  of  operation  includes  wages,  cost  of  power, 
water,  light  and  lubricants.  In  regard  to  the  cost  of  power  and 
water  we  must  consider  that  they  depend  in  many  cases  on  the 
more  or  less  perfect  operation  and  efficiency  of  the  machinery. 
An  increase  in  the  cost  of  power  and  water,  if  it  brings  about,  a 
cleaner  washed  coal,  is  commendable  if  this  increase  remains 
below  the  possible  better  price  obtained  for  the  cleaner  product. 

The  cost  of.  special  work  includes  wages  and  cost  of  material 
for  repairs  and  renewals.  While  the  above-named  cost  can  at 
least  partly  be  predetermined,  that  of  repairs  appears  only  in 
the  course  of  time,  after  the  washers  have  been  in  operation.  To 
arrive  at  the  exact  cost  of  repairs  is  difficult.  Depending  upon 
the  time  used  for  repairs,  the  absolute  expense  is  much  higher 
than  the  cost  of  labor  and  material  expended,  because  we  must 
take  into  account  the  loss  incurred  through  the  interruption  of 
operation  of  the  washery,  which  may  in  some  cases  reflect  even 
upon  the  operation  of  the  mine. 

The  breaking  down  of  an  elevator,  with  the  bins  full  and  no 
spare  parts  on  hand,  may  be  given  as  an  example.  Therefore, 
all  important  machinery  ought  to  be  fully  guaranteed  by  re- 
sponsible manufacturers  as  a  safeguard  against  interruption  of 
operation.  This  may,  however,  bring  about  an  increase  in  the 
cost  of  installation,  influenced  by  the  heavier  and  better-con- 
structed machinery. 

The  cost  of  washing  coal  shows  just  as  many  variations  as 
everything  else  connected  with  a  washery.  The  following  fig- 
ures (Table  56),  however,  can  be  given  as  an  approximate  guide: 


356  COAL  WASHING 


Cost  per  Ton  of  Raw  Coal 

Minimum,       Maximum, 

Cents  Cents 


Amortization  and  interest  of  capital  invested 3  5 

Cost  of  operation  (wages,  power,  water,  light,  stores)  8  20 

Cost  of  repairs 2  5 

Total 13  30 

TABLE  56 


To  the  foregoing  figures,  however,  must  be  added  the  cost  of 
shrinkage,  which  will  depend  upon  the  amount  of  impurities  in 
the  raw  coal  and  the  degree  of  cleaning — that  is,  upon  the  yield. 
I  have  operated  different  washers  that  made  from  10  per  cent,  to 
33  per  cent,  refuse. 

L.  A.  0.  Gabany  gives  the  cost  of  washing  in  Alabama  as 
follows : 

STEIN  JIGS  IN  THE  FIRST  YEAR  OF  INSTALLATION;  ELEVEN  JIGS  AND  THREE 
OPERATORS  FOR  THREE  CONSECUTIVE  DAYS 


Run 
Ash 
per 
cent. 

of  Mine 
Sulphur 
per 
cent. 

Washed  Coal 
Ash       Sulphur 
per            per 
cent.         cent. 

Refuse 
Content 
Per  Cent. 
Good       Bone 
Coal 

Output       Cost 
for    Three  per  Ton. 
Days      of    Coal 
Tons        Cents 

Analysis 
Ash 
per 
cent. 

of    Coke 
Sulphur 
per 
cent. 

12.91       1.44         5.0         0.86        3.24        2.62       1,646       5.32        8.96        0.96 

3  jig  tenders  at  $3  each. . $  9.00 

3  assistant  tenders  at  $2  each 6.00 

3  oilers  at  $1  each 3.00 

Oil    2.10 

Water 9.05 

Fuel 6.55 

Repairs 2.50 

$38.20 

2.32  cents  per  ton 
3.00  depreciation 

5.32  cents  total,  irrespective  of  the  good  coal  lost  in  10.4  per  cent, 
refuse  which  equals  10  tons  of  good  coal  in  three  days. 

TABLE  57 


F.  C.  Lincoln,  in  Bulletin  No.  69  of  the  Engineering  Experi- 
ment Station  of  the  University  of  Illinois,  tabulated  the  cost  of 
washeries  and  of  washing  for  Illinois  washeries  as  shown  in 
Table  59.  In  explanation  of  this  table  Mr.  Lincoln  says : 


COST  OF  WASHING  COAL  357 

STEIN  JIGS  FOB  THE  FIFTH  YEAR  FROM  INSTALLATION;   ELEVEN  JIGS  AND 
THREE  OPERATORS  FOR  Two  DAYS 


Run 
Ash 
per 
cent. 

of  Mine 
Sulphur 
per 
cent. 

Washed  Coal 
Ash       Sulphur 
per           per 
cent.         cent. 

Refuse 
Content 
Per  Cent. 
Good       Bone 
Coal 

Output 
for  Two 
Days 
Tons 

Cost    Analysis 
per  Ton     Ash 
of    Coal      per 
Cents       cent. 

of    Coke 
Sulphur 
per 
cent. 

13.20 

1.32 

8.15 

0.86 

24.9 

16.2 

1,046 

5.88 

11.22 

0.98 

2 
2 
2 

jig  tenders  at  $3 
assistant  tenders 
oilers  at  $1   each 
Oil    

each 

$ 

600 
4.00 
2.00 
1.40 
6.06 
4.27 

at  $2 

each 

Water    . 

Fuel    . 

Repairs    6.40 

$30.13 

2.88  cents  per  ton 
3.00  depreciation 

5.88  cents  total,  irrespective  of  good  coal  lost  in   10.7   per  cent, 
refuse  which  equals  34.83  tons  of  good  coal  in  two  days. 

TABLE  58 

"The  costs  of  power,  labor,  supplies,  repairs  and  renewals  per  ton  of  raw 
coal  washed,  as  reported  by  fifteen  washeries,  varied  from  3  to  18  cents, 
with  an  average  of  about  10%  cents.  In  obtaining  this  average,  depreci- 
ation was  included  in  one  instance  but  this  is  probably  more  than  offset 
by  omission  of  costs  of  power  from  reported  general  washing  costs  which 
are  likely  to  be  made  when  mine  and  washery  are  operated  with  the  same 
power  plant,  so  it  is  believed  that  this  average  is  low  rather  than  high. 
The  cost  of  building  fifteen  washeries  with  a  combined  capacity  of  1740 
tons  raw  coal  per  hour  was  $572,000.  At  the  same  rate  the  33  operating 
commercial  washeries  of  Illinois  with  their  combined  hourly  tonnage  of 
3555  would  have  cost  $1,169,000.  These  figures  do  not  represent  the  total 
investment,  in  washeries,  as  they  do  not  include  cost  of  land  for  washery, 
reservoirs  and  refuse  dump  sites.  Costs  of  individual  washeries  could  not 
be  included  in  Table  59  without  violating  the  confidences  of  some  of  the' 
operators,  but  costs  per  ton  rated  capacity  are  given;  These  costs  are  for 
washeries  with  hourly  tonnages  ranging  from  25  to  280  and  averaging  116, 
and  show  costs  of  from  $130  to  $583  per  ton  capacity  per  hour,  with  an 
average  of  $351.  The  costs  of  individual  washeries,  while  showing  consider- 
able irregularity,  still  vary  in  a  general  way  with  size  and  type.  The 
average  cost  per  ton  capacity  per  hour  of  seven  washeries  with  capacities 
of  100  tons  and  under  was  $448,  while  eight  washeries  with  capacities  in 
excess  of  100  tons  averaged  $266.  The  average  cost  of  three  Liihrig  jig 
washeries  was  $393  per  ton  capacity  per  hour,  of  six  Stewart  jig  washeries 

The  Elmore  washery  shown  in  Fig.  202  did  cost  $677  per  ton  rated 
hourly  capacity. 


358 


COAL  WASHING 


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CHAPTER  XXXVII 

GENERAL  ARRANGEMENT  OF  WASHERIES  AND  GRAPHI- 
CAL ILLUSTRATION  OF  THE  PROCESS 

The  design  of  a  coal  washery  is  a  complicated  problem  on  ac- 
count of  the  extremely  numerous  factors  influencing  the  arrange- 
ment. This  becomes  still  more  complicated  when  the  separate 
requirements  become  contradictory.  We  have  in  the  foregoing 
chapters  fully  discussed  which  requirements  should  be  considered 
and  which  should  be  given  preference.  Drawings  for  a  washery 
can  be  made  in  different  ways,  depending  upon  the  purpose  for 
which  they  are  intended. 

If  it  is  only  necessary  to  get  an  idea  of  the  methods  used  and 
the  succession  of  the  operations,  plain  flow  sheets  will  suffice. 
Flow  sheets  are  of  great  help  for  preliminary  estimates.  They 
are  indispensable  when  the  operations  become  complicated,  in 
order  to  comprehend  quickly  the  correlation  of  the  different 
processes.  In  Figs.  189  and  190  two  flow  sheets  are  shown.  One 
for  a  fuel-coal  washery  taking  3  in.  screenings  from  a  distant 
mine  and  the  other  for  a  coking-coal  washery  directly  connected 
with  the  mine.  The  flow  sheet  for  the  fuel-coal  washery  illus- 
trates the  operation  of  the  washery  shown  in  Fig.  197. 

In  Fig.  188  a  different  kind  and  more  elaborate  type  of  flow 
sheet  is  shown  for  a  washery  making  five  sizes  of  fuel  coal  and 
a  coking  coal  at  the  same  time.  In  this  flow  sheet  the  different 
pieces  of  machinery  are  shown  in  outline  and  the  separate  units 
are  shown  in  nearly  the  same  juxtaposition  as  they  are  placed 
in  the  washery. 

In  studying  this  flow  sheet  we  find:  That  the  dry  screened- 
off  dust  can  be  mixed  directly  with  the  washed  fine  coal.  The 
middle  products  from  the  coarse  coal  jigs  can  be  carried,  accord- 
ing to  their  composition,  either  back  to  the  coarse  coal  jigs  or 
to  the  rewash  jigs.  In  the  latter  case  only  boiler-house  coal  can 
be  made.  If  the  amount  of  fine  coal,  screened  out,  is  not  suffi- 

359 


360 


COAL  WASHING 


FLOW  SHEETS 


361 


SCREEN  WITH 3- HOLES}  \TRAM 

_j& —     — zs^1 


DRIED  AND  WASHED    HOUSE  GOAL 
COAL 

Fig.  189.     Flow  Sheet  for  Coking  Coal  Washery 


362 


COAL  WASHING 


cient  for  the  supply  of  the  coke  ovens,  some  of  the  nut  coal  can 
be  crushed  and  delivered  in  connection  with  some  foreign  fine 
coal  to  the  coking  coal  bins. 

In  the  sludge  cistern  the  following  materials  are  collected: 
(a)  The  drained-off  water  from  the  fine  coal;  (b)  the  sludge 
from  the  clearing  tanks  after  being  filtered  through  the  screens ; 


[  3'5CRE£NrNGS  FROM  MINE  IN  R.  R.  CARS  \ 


ff  UN  LOADER} 
' 


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I      ~FRESH_  WTER^ 


RAfLROAD     CARS 

Fig.  190.     Flow  Sheet  for  Fuel  Coal  Washery  Shown  in  Fig.  197 

(c)  the  overflow  water  from  the  fine  coal  bin;  (d)  the  water 
drained  off  from  the  crushed  nut  coal;  (e)  the  overflow  water 
from  the  boiler  house  coal  storage  bin. 

The  screw  conveyors  over  the  coking  coal  bins  are  used  to  mix 
the  fine  coal,  the  dry  dust  and  the  crushed  nut  coal  with  the  for- 
eign fine  coal.  The  first  clearing  tank  produces  sludge,  which 


FLOW  SHEETS 


363 


can  be  used,  but  the  second  tank  only  during  continuous  opera- 
tion, as  after  a  shut-down  the  fireclay  settles  out  on  the  bottom 
and  must  be  removed  to  the  clearing  basins. 

The  flow  sheet,  Fig.  191,  shows  the  progress  of  operation  for 


RUN  OF  ntNE&  NUT  COAL 


EIWAUSTFAN 
_  —  .-  —  _  -  - -t 


UNXR3ZE    OVERSIZE 
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Fig.  191.     Flow  Sheet  for  Washery  Illustrated  in  Figs.  198  and  199 


a  coking-coal  washery  arranged  according  to  Figs.  198  and  199. 
This  washery  is  arranged  to  take  coal  from  several  mines.  Run- 
of-mine  is  received  in  railroad  cars  and  dumped  in  a  track  hop- 
per. From  this  hopper  the  coal  is  passed  over  a  screen.  The 


364 


COAL  WASHING 


Vibrating 
Screens 

Vibrating 
Shaker 
Screens 
60  "x  200  " 

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for    Deliver;    *f    Coal    aj    Follow!  : 
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Coarsa    Coal    to    Convejor  J. 
All    3    Blxes    to    Conveyor  Q. 
rinea    &    luteruedlate,    Cc»v.  H  or  Q. 
CoarM  &            „          ,       „       G  or  J. 

Fig.  192.     Flow  Sheet  for  Coking  Coal  Washery  Illustrated  in  Fig.  200 


FLOW  SHEETS 


365 


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COAL  WASHING 


FLOW  SHEETS 


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oversize  from  the  screen  is  crushed  and  the  crushed  and  screened 
coal  is  put  into  a  storage  bin,  which  also  receives  the  screenings 
from  other  mines.  From  the  storage  bin  the  coal  is  conveyed 
to  the  equalizing  bin,  located  in  the  rear  of  the  jigs.  Prom  here 
feeders  carry  the  coal  to  the  jigs.  At  the  feeders  the  dust  col- 


lected  at  the  screen  house  is  mixed  in  with  the  coal.  The  jigs 
are  three-compartment  machines,  making  three  products,  which, 
depending  on  their  composition,  can  be  treated  in  different  ways. 
From  the  washed-coal  settling  tank  the  coal  is  conveyed  to  a 
series  of  draining  bins  to  be  dewatered,  and  from  these  bins  it  is 


372 


COAL  WASHING 


conveyed  to  the  coke-oven  larry  bins  and  thence  to  the  coke  ovens. 
The  refuse  is  deposited  in  a  refuse  bin  and  carried  away  in  rail- 
road cars  to  a  dump.  The  recovery  coal,  after  passing  over  a 
draining  or  dewatering  conveyor,  is  stored  in  a  bin. 


The  dirty  water  from  the  recovery  coal  draining  conveyor,  the 
overflow  water  from  the  washed-coal  settling  tank,  the  wash-out 
water  from  the  jig  tanks  and  the  washed-coal  settling  tank,  and 
the  drip  water  from  the  draining  bins  is  collected  in  a  recovery 


FLOW  SHEETS 


373 


spitzkasten.  The  settlings  from  this  spitzkasten  are  further 
treated  on  a  recovery  screen  and  the  resulting  recovery  coal 
mixed  in  with  that  coming  from  the  jigs.  The  cleared  water  is 
collected  in  a  cistern  for  reuse.  The  drip  water  from  the  drain- 
ing bins  can  also  be  conveyed  back  to  the  washed-coal  settling 
tank. 

The  dirty  water  from  the  refuse  draining  conveyor  and  the 
recovery  screen  is  treated  in  a  clearing  spitzkasten.  The  set- 
tlings pass  over  a  mud  screen.  The  resulting  mud  is  mixed  with 
the  outgoing  refuse  and  the  dirty  water  from  the  screens  car- 
ried back  to  the  clearing  spitzkasten.  The  cleared  water  from 


Fig.  201.     Loading  Arrangement  of  Washery  Shown  in  Fig.  200 

the  spitzkasten  flows  to  the  clear-water  cistern.  The  circulating 
pump  takes  the  water  from  the  washed-coal  settling  tank  and 
puts  it  back  under  the  jigs.  Fresh  water  is  supplied  to  the  dust 
collector,  the  jigs,  the  recovery  and  the  mud  screens. 

The  general  arrangement  of  a  fuel-coal  washery  is  shown  in 
Fig.  197.  Flow-sheets,  Figs.  193  to  196,  were  supplied  by  the 
Engineering  Experiment  Station  of  the  University  of  Illinois 
from  Bulletin  No.  69,  "  CoaTWashing  in  Illinois,"  by  F.  C. 
Lincoln. 

Fig.  200  shows  the  general  arrangement  of  a  modern  coking 
coal  washery,  taking  either  run  of  mine  direct  from  the  tipple  or 
foreign  coal  by  means  of  an  unloading  pit.  This  washery  is 
equipped  with  an  elaborate  system  for  clarifying  the  water  and 
recovering  the  sludge.  The  washed  coal  is  dewatered  in  Elmore 
centrifugal  dryers  and  loaded  directly  into  railroad  cars..  This 


374 


COAL  WASHING 


FLOW  SHEETS  375 

method  avoids  the  construction  of  a  costly  washed  coal  storage 
bin.  Fig.  201  shows  the  loading  arrangement,  which  consists 
only  of  two  chutes,  one  over  each  of  the  two  loading  tracks. 

Fig.  192  gives  the  flow  sheet  for  this  washery,  and  Figs.  186 
and  187  show  this  washery  in  course  of  construction. 

Fig.  202  illustrates  the  general  lay-out  of  a  coking  coal  wash- 
ery taking  coal  direct  from  the  mine  by  means  of  a  belt  con- 
veyor. This  washery  is  equipped  with  two  three-compartment 
"Elmore"  jigs  and  one  rewash  jig.  The  sludge  recovery  is  car- 
ried on  in  an  elevated  conical  tank. 


INDEX 


Air  separators,  50 
Air  valve  for  Baum  jig,  197 
Alabama  docking  schedule,  76 
Althans  review  of  the  -status  of  coal 

washing.  8 
American  filter,  315 
American  jig,  200 
Analysis,  of  coal  physical,  135 
Apron  feeder,  160 

Arrangement   of   coking  coal   wash- 
cry,  370,  372,  374 

fuel  coal  washery,  369 

motors  and  drives,  344 

washeries,  74,  359 
Artificial  bed  for  jigs,  181 
Artois  sludge  treatment,  70 
Automatic  jig  control,  178 
Ash  contents  of  raw-coal,  dust  and 

slime,   104 
Ash  and  sulphur  elimination,  154 


Bache  sludge  tank,  66 
Bangert  washer,  48 
Baum  jig,  198 

air  valve  for,  197 
Basins  clearing,  results  with,  289 
Basket  type  jigs,   199 
Bell's  trough  washer,  47 
Belt  conveyors,  88 
capacity  of,  88 
Berard's  differential  motion,  36 

jig,  37 
Bin,  draining,  272 

equalizing,   160 

fine  coal,  265 

nut  coal,  240 

raw-coal,  85,  91 
Bradford  breaker,  58,  253 
Bring  on  jigging,  25 
Bucket  elevators,  construction  of,  94 
Buildings  and  structures,  348 
Butchart  concentrating  table,  217 

results,  223 

Cadiat  centrifugal  washer,  52 


Cast-iron  jigs,  212 
Carr  disintegrator,  13 
Centrifugal  dryers,  274 
Elmore,  275 
data,  281 
Hanrez,  61,  283 
separators,  51 
washer,  Cadiat,  52 
Chemical  survey  of  a  mine,  136 
Clarification  of  water,  64,  286 
Classification,  Remy,  9 
Classifier,  Dorr,  292 

Evard,  43 

Classifying  of  coal,  8,  103 
Cleaner  coal,  demand  for,  2 
Clearing  basins,  results  with,  289 
Coal  dust,  use  of,  109 
Coal,  occurrence  of  pyrite  in,  321 
Coal,  preparation,  benefit  of,  2 
Coal  washing,  methods  of,  157 
conclusions  on  methods  of,  59 
review  of  status  of  Althans,  8 
Coking  coal  washery,  arrangement, 

370,  372,  374 
flow  sheets,  361,  363,  364 
Compressed  air  for  conveying  sludge, 

287 

jigs,  197 

Concentrating  tables,  215 
advantages  of,  233 
Butchart,  217 
cost  of  installation,  233 

operation,  233 

data,  216,  223,  226,  227,  234 
Deister-Overstrom,  224 
Deister  Plat-O,  227 
head  motion  for,  225 
Massco,  216 

Morrow,  J.  B.,  opinion  on,  230 
Overstrom  Universal,  229 
power  forr  233 
results  with,  223 
riffle  construction  of,  215 
Concrete  jig  tanks,  212 
settling  tank,  291 
raw-coal  bin,  93 


377 


378 


INDEX 


Construction  of  elevators,  94 

jigs,  212 

raw-coal  bins,  91 

riffles,  215 

a  washery,  348 

Control;  automatic  jig,  Elmore,  178 
Conveying   sludge   with   compressed 

air,  287 
Conveyors,  belt,  88 

raw-coal,  87 

scraper,  88 

Cost  of  table  installation,  233 
operation,  233 

washeries,  357 

washing,  354 
in  Alabama,  356 

Illinois,  358 
Crusher  data,  263 

gyratory,  13 

jaw,  13 

hammer,  248 

needle,  13,  244 

types  of,  243 

roll,  13,  246 
Crushing,  Drakeley  on,  242 

Gaetzschman  on,  11 

Hancock  on,  12 

Meynier  on,  *11 

De-Francey  and  Jarlot  washing  ma- 
chine, 39 
Deister-Overstrom  table,  224 

data  on,  226 
Deister   Plat-O  coal   washing  table, 

227 

capacity  of,  227 
Dense  liquid  separation,  57 
Design  of  a  washery,  353 
Dewatering  elevators,  274 
of  fine'  coal,  265 
pits,  267 

capacity  of,  270 
of  washed  nut  coal,  236 
Differential   drive   for  jig  plungers, 

167 

motion,  Berard,  36 
Ding's  magnetic  pulley,  123 
Disintegrator,  Carr,  13 

Stedman,  13 

Docking  rule,  importance  of;  77 
schedule  in  Alabama,  76 
system,  6 


Dor  apparatus  for  treating  sludge,  68 
Dorr  classifier,  292 
thickener,  67,  286,  290 

Campbell,  J.  R.,  on,  299 

flow  sheets,  293,  298,  300 

installation,  293 

operation,  294 

results  with,  296 
Draining  bins,  272 
Drive,  electric  motor,  344 
Drum  feeder,  '160 
Dry  separation,  6 
Dryer  centrifugal,  274 
Dryer    for    washed    coal    by    Riehn, 

Meinicke  and  Wolf,  62 
Drying  coal,  methods  of,  267 
Drying  of  sludge,  304 

Campbell,  J.  R.,  on,  305 

with  filter,  305 

Lowden  dryer,  318 
Dust,  ash  contents  of,  104 
removal,   106 

Eccentric,  adjustable,   165 
Effect  and  cause  in  jigging,  211 
Efficiency   calculation   of   a   washer, 
151 

chart  by  Hancock,  144 

general,  153     .    : 

qualitative,  151 

quantitative,  151 
Electric  motor  drive,  344 
Elevator  capacity,  102 

construction,  94 

dewatering,  274 

hold  back,  99 

raw-coal,  94 

Elimination  of  ash  and  sulphur,  '154 
Elliot  trough  washer,  48 
Elmore  automatic  jig  control,  178 

centrifugal  dryer,  275 

jig,  168 
Evard  classifier,  43 

jig,  42,  44 

Evolution  of  the  jig,  31 
Equalizing  bins,  160 
Equal  settling,  law  of,  16 
Eccentric,  adjustable,  165 

Faust  jig,  188 
Feeder,  apron,  160 
drum  type,  160 


INDEX 


379 


Feeder  —  continued 

mechanically  operated,  160 
shaking,  1GO 
Feeding  of  jigs,  160 
Feldspar  bed  for  jigs,  183 
Filter,  American,  315 
for  drying  sludge,  305 
Oliver,  313 
Portland,  306 
Zenith,  308 

Fine  coal  storage  bin,  265 
Flanchon  jig,  40 
Flow  sheets  for  coking  coal  washer- 

ies,  361,  363,  364 
combined  coking  and  fuel  coal 

washery,  360 
Dorr      Thickener      installation, 

293,  298,  300 
Fuel-coal  washery,  362 
Liihrig  washery,  367 
Pyrite  recovery  plant,  326 
re-crushing  Jig  middlings,  327 
Robinson-Ramsay  washery,  365 
Shannon-Faust  washery,  368 
Stewart  washery,  366 
water    clarification    and    sludge 

recovery  system,  293,  300 
Formula   for    determining   the    per- 
centage  of   solids   in   pulp, 
299 

Forrester  jig,  163 
Fuel  coal  washery,  arrangement  of, 

369 
Fuel  supply,  economic  question  of,  3 

General  efficiency,  153 

Gervais  jig,  40 

Girard  jig,  40 

Graphs,  Pascal's,  146 

Guarantee  for  the  performance  of  a 

washery,  156 
Gyratory  crusher,  13 

Hammer  crushers,  248 

Hand  operated  jigs,  31 

Hanrez  centrifugal  dryer,  61,  283 

Harman  sludge  treatment,  72 

Head  motion  for  concentrating  tabes, 

Deister  heavy  duty,  225 

Overstrom,  226 

Hindred  settling  conditions,  24 
Hold  back  for  elevators,  99 


Installation  of  concentrating  tables, 

cost  of,  233 
Investigation,  preparatory,  133 

Jaw  crushers.  13 
Jig  action,  23,  26 
Jig,  American,  200 

with  artificial  bed,  181,  183 

basket  type,  199 

Baum,  198 

Berard,  37 

capacity  of,  207 

cast-iron,  212 

compressed  air,  197 

construction,  212 

control  of,  208 
automatic,    178 

with  differential  plunger  motion, 
167 

Elmore,  168 

Evard,  42,  44 

evolution  of,  31 

Faust,  188 

feeding  of,  160 

Feldspar  bed  for,  183 

Flanchon,  40 

Forrester,  163 

Gervais,  40 

Girard,  40 

hand  operated,  31 

Kasselowsky,  195 

Lacratelle,  35 

location  of,  213 

Liihrig,  42 
fine  coal,  184 
nut  coal,  162 

Marsais,  40 

mechanical  arrangement  of,  143 

Meynier,  38 

Montgomery,  187,  195 

Neuerburg,  42 

operation  of,  208 

Pittsburg,  201 

plunger  type,  162 

with  plunger  below  screen,  186 

power  required  for,  207 

products,  treatment  of,  235 

Raxt-Madoux,  40 

Revolier,  42 

Rexroth,  42 
.      Robert,  40 

Shannon,  204 


380 


INDEX 


Jig  —  continued 

Sievers,  42 

steel  plate,  212 

Stewart,  199 

tank  concrete,  212 
types,  35 

three  compartment,  190 

timber,  212 

two  compartment,  191 

types,  161 

work,  scope  of,  29 

tables  on,  by  Rittinger,  21 
theoretical  foundation  of,  19 
Jigging,  effect  and  cause,  211 

opinion  on,  Sparre,  27 

process,   15 

study  of  by  Richards,  29 
theory  by  Rittinger,  23 

Kasselowsky  jig,  195 
Kettle  valve,  191,  213 
Knowledge   gained   by   past   experi- 
ence, 10 

Kohl-Simon  sludge  treatment,  302 
screen,  results  from,  303 

Lacratelle  jig,  35 

Law  of  equal  settling,  16 

Location  of  jigs,  213 

sludge  recovery  plant,  287 

storage  bins,  87 
Lombard  washing  machine,  40 
Lowden  dryer,  318 
Liihrig  jig,  42,  162,  184 

Magnetic  pulley,  Dings',  123 

separator,  124 
Marsais  jig,  40 

Massco  coal  washing  table,  216 
Maxims  for  coal  washing,  4 
Mechanical  arrangement  of  jigs,  143 
Mechanically  operated  feeders,  160 
Merrick  Weightometer,  127 
Methods  for  calculating  percentage 

of  washed  coal  and  refuse, 

154 

Meyiiier  jig,  38 
Middle  products,  262 
Mine-run  sampler,  79 
Montgomery  jig,  187,  195 
Motors  and  drives,  344 


Needle  crusher,  13,  244 
Nickle-Plate    seam,    report    on,    by 

Hancock,  137 
Nut  coal  storage  bins,  240 


Oliver  filter,  313 

Operation  of  Dorr  thickener,  294 
results,  296 

a  jig,  208 

concentrating  tables,  cost  of,  233 
Opinion  on  the  action  in  a  jig,  25 

screening,  7 
Overstrom  "Universal  Table,"  229 


Parrish  screen,  120 
Pascals'  graphs,  146 
Percentage    of    solids    in    pulp,    for- 
mula  for  determining,  299 
washed  coal  and  refuse,  method  of 

calculating,  154 
Performance  of  a  washer,  144 

guarantee  for,  156 
Pits,  dewatering,  267 
Pittsburg  jig,  201 
Physical  analysis  of  coal,  135 
Plunger  speed,  209 
stroke,  209 
type  jig,  162 
Portland  filter,  306 
Power,  342 

for    the    different    pieces    of    ap- 
paratus in  a  washery,  343 
jigs,  207 
required  per  ton  of  coal  washed, 

342 
for  concentrating  tables,  233 

water  circulation,  341 
Preparation,  mechanical,  of  screened 

coal,  6 
Preparatory  investigations,  133 

sizing,  110 

Proceedings  at  the  mine,  75 
Products  of  jigs,  treatment  of,  235 
Purpose  of  de-watering  fine  coal,  265 

washing  coal,  1,  2 
Pyrite  in  coal,  321 
recovery,  320 

plant,  flow  sheet  for,  326 

tests  by  E.  A.  Holbrook,  323 
in  Tennessee,  333 


IKDEX 


381 


Qualitative  efficiency,  151 
Quantitative  efficiency,  151 

Ract-Madoux  jig,  40 

Ramsay,  Mine-run  sampler,  ?9 

sludge  tank,  54,  65 
Raw-coal,  ash  contents  of,  104 
bins,  91 

capacity  of,  91 
concrete,  93 
construction  of,  91 
conveyor,  87 
elevator,  94 
supply,  208 
Recovery  of  pyrite,  320 

sludge,  286 

Reduction  of  ash  and  sulphur,  155 
Refuse  discharge,  210 
Removal  of  dust,   106 

screens  for,  107 
tramp  iron,  123 
Requirements  for  a  crushing  plant, 

13 

Revolier  jig,  42 
Revolving  slate  gate,  213 
screens,  capacity  of,  238 

types,  117 

Rewashing  of,  middle  products,  262 
Rewashing,  process  of,  263 
Rexroth  jig,  42 

Rhum  belt  washing  machine,  49 
Rhien,     Meinicke     and     Wolf     coal 

dryer,    62 

Riffle  construction,  215 
Robert  jig,  40 

Robinson-Ramsay  washer,  53 
Roll  crushers,  13,  246 

results  of  test  with,  247 


Sampler  for  mine-run  coal,  79 
Sampling  apparatus,  129 
Schmitts'  air  separator,  50 
Scope  of  jig  work,  29 
Scraper  conveyors,  88 
Screening  of  coal,  7 

opinions  on,  7 
Screens,  115 

demands  made  upon,  113 
for  dust  removal,  107 
Parrish,  120 


Screens  —  continued 
revolving,  116 

capacity  of,  238 
shaking,  118 
Separators,  air,  50 
centrifugal,  51 
magnetic,  124 

Separation  with  dense  liquids,  57 
dry,  6 
wet,  15 
Setfling  conditions,  16,  24 

hindered,  24 
tank,  concrete,  291 
Shaking  feeder,  160 

screens,  118 
Shannon  jig,  204 
Sievers  jig,  42 
Sizing  of  coal,  105,  110 
preparatory,  110 
after  washing,  111 
Slate  gate,  213 
Slime,  ash  contents  of,  104 
Sludge,   conveying  with  compressed 

air,  287 
drying  of,  304 
tank,  Bache,  66 

Ramsay,  54,  65 
treatment,  302 
Artois,  70 
Bache,  66 
Dor,  68 
Harman,  72 
Karlik,  72 
Kohl-Simon,  302 
Zorner,  72 
recovery,  286 

plant,  location  of,  287 
Spitzkasten  clearing  basins,  results 

from,  289 

Speed  of  jig  plungers,  209 
Stedman  disintegrator,  13 
Steel  plate  jig,  212 
Stewart  jig,  199 
Storage  bin  for  raw-coal,  85 

location  of,  87 
fine  coal,  265 
for  washed  nut  coal,  240 
Stroke  of  jig  plungers,  209 
Sulphur  in  coal,  142 
Supply  of  raw-coal,  208 

water,  208 
Survey,  chemical  of  a  mine,  136 


382 


INDEX 


Tables,  concentrating,  215 
installation  of,  233 
power  required  for,  233 
Tanks,    jig,    concrete    construction, 

212 
types  of,  35 

settling,  66 

concrete  construction,  291 

sludge,  54 
Theory  of  jigging  by  Rittinger,  23 

wet  separation,  15 
Thickener,  Dorr,  67,  286,  290 
Three-compartment  jig,  190 
Timber  jig,  212 
Tramp  iron,  removal  of,  12-3 
Treatment  of  jig  products,  235 

sludge,  68,  302 

washed  nut  coal,  236 
Trough  washer,  46 
Trough  washer,  Bell,  47 

Elliot,  48 

Two-compartment  jig,  191 
Types  of  crushers,  243 

jigs,  161 

washing  methods,  157 


Value  of  washed  coal,  134 

washing  coal,  1 
Velocities,  table  of,  by  Sparre,  18 


Washability  of  a  coal,  by  Eraser  and 
Yancev,   146 


Washed  coal  discharge,  213 

storage,  240 

nut  coal  treatment  of,  236 
Washer  efficiency,  calculation  of,  151 
Washeries,  arrangement  of,  74,  359 
construction  of,  348 
cost  of,  357 
design  of,  353 
flow  sheets  for,  360 
performance  of,  144 
Washing,  cost  of,  354 
methods  of,  157 

conclusions  on,  59 
process,  control  of,  208 
tests,  135 

results  of,  138 
Water,  circulation  of,  341 
clarification,  64,  286 

flow  sheets  for,  293,  300 
losses,  340 
requirements,  339 
supply,  208 
system,  339 
Weighing  apparatus,  127 

importance  of,  129 
Weightometer,  Merrick,  127 
W7ork  of  jigs,  scope  of,  29 

theoretical  foundation  of,  19 

Yield  of  a  washery,  133 

Zenith  filter,  308 

rotary  hopper  dewaterer,  312 
Zorner  sludge  treatment,  72 


VC  34034 


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